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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Mon 28, 2019 4:45 am 
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I'll try that.

May take the amp and experiment with that as I can just disconnect the wiper of the 20K resistor and feed a HI-FI signal to the 4.7K resistor and see how it sounds.

If it sounds good enough for HI-FI without the .001uF cap I'll order the parts to build a HI-FI version while keeping this one for the bass.

EDIT:

Tested the amp and it is only suitable for bass duty.

At around 600Hz with the capacitor removed there started to be a part at the zero crossing point of the waveform that looked a lot like the waveform an unproperly biased push pull amp makes. As I went closer to 16KHz the waveform started to look more triangular.

So the amp is no good for HI-FI.

That said one look at the amp board itself could have told me that because if the amp was suitable for HI-FI they would have used it for the other amps and saved two transistors two pots and a few resistors.

I did find out that varying the 470K resistor does affect gain down to no output at 0 ohms and it doesn't seem to affect the DC offset either.

I may try adding a second filter just to see how it sounds then I can remove the 20K pot and use a 500K trimpot for the gain control.

That said if I add a second filter it will also have a voltage gain.

I still may try the quirk of the Philco putting lower frequency audio voltage on the B+ line the driver transformer connects to though as that would provide a low pass pretty much matching the high pass formed by that filter cap being too small a value to adequately filter out all audio from the B+ line.

Now if that quirk were to work I'd use it for sure.

That said I do notice a slight hum from the Philco's speaker and I am not sure if it was there before I removed the larger cap across the 1uF cap (49), but I can test that by putting the cap across the 1uF cap and see if the hum goes away.

EDIT 2: Didn't like how warm the bridge rectifier was getting so I swapped it out with four diodes.

DC voltage no load starts out at 42.16Vdc and full load at a 40Hz frequency into four ohms is 36.5Vdc.

Part of that is the transformer voltage dropping by 1.75Vrms, but that isn't the whole reason.

I'm thinking it is the filter caps not being quite large enough. I have a couple larger value caps I can parallel with the originals. Should take care of it.


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Wed 30, 2019 6:03 am 
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Well then we have success. :D :D :D

I didn't have a 90K or 91K and 10K resistor so I used two 39K and one 22K for testing.

Connected the amp to the 22K resistor with the other end of the resistor being connected to the filter cap using a .22uF cap between the resistor and amp.

Works quite well.

I had the .22uF cap in place of the .015uF cap and I replaced it with a .01uF cap. Still need a lower value of cap.

So based on this it looks like a .002uF cap would work

Attachment:
002 cap.png
002 cap.png [ 41.46 KiB | Viewed 1082 times ]


I will replace the resistors with a 91K and 10K or whatever will get me closest to 100K with no more than 10K for the resistor the signal is taken from.

I do indeed have a slight hum which I need to deal with and might be that 1uF filter cap being too small so perhaps I'll reinstall the larger value electrolytic.

EDIT:

The hum isn't from the power supply.

The first audio tube is currently unshielded as all but one tube shield was missing when I got it.

Perhaps that is the source?

I do know that is causing audio to be present in the speaker when the volume control is full CCW as I can put my hand around the tube and the audio starts to get lower.

Now the .22uF cap works, but is too large as when the bass amp is on and I turn on the radio it makes a couple hums in the audio.

I can use a .01uF cap there, but I get a phase shift starting around 160Hz which is shifted 20 degrees at 40Hz.

Now if I use a .068uF cap the phase shift starts at 60Hz and is shifted a little below 5 degrees at 40Hz. That would work, but I'd possibly still get the brief hum.

Any way to make it to where the brief two hums doesnt happen?


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Wed 30, 2019 4:11 pm 
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Quote:
Any way to make it to where the brief two hums doesnt happen?
I'm trying to follow along based on your descriptions of the radio schematic and how you've tapped off audio. You've not posted a partial schematic of it. If I understand, you hear a couple of hums from the subwoofer amp when you turn on the radio. When the hums occur (120Hz presumably), I am imagining the Jenson speaker is pulled in one direction then pushed in the other (or vice versa), and the hums comes from current draw off the sub's power supply to push/pull the speaker. Does that all sound right?

Your coupling cap has DC potential across it during operation, positive on the radio side and ground on the sub amp's side, and when it charges up with radio turn-on, it imparts a huge low frequency pulse to the sub amp. There are two hums because the speaker swings twice, once on the up side and once in the opposite direction at the return. (The couple caps within the sub amp turn one long pulse into two shorter ones.) Either mute the subwoofer amp (delay its turn on) or figure a way to VERY slowly ramp the radio's +B supply.

BTW, some phase shift at the lowest frequencies would not be a worry of mine! No one, including those people who revel in their seat-shaking home theaters with multiple subwoofers, will hear the phase shift.

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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Wed 30, 2019 4:52 pm 
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That's exactly what is going on.

My plan is to update the amp schematic with where I'm taking the audio from in the radio.

What I wonder is this.

Given the high input impedance of the buffer could I insert a .001uF or smaller cap between the wiper of the 1 meg pot and + input pin of the OP-AMP and if so would that reduce the problem?

If not I can always use a NC delay relay set for the time it takes the tubes to warm up using that to short the input to the amp.


EDIT:

Updated schematic.

Attachment:
Bass amp 6.png
Bass amp 6.png [ 54.01 KiB | Viewed 1072 times ]


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Wed 30, 2019 6:48 pm 
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The turn-on pulse (coupling cap charging up) looks exactly like low frequency audio to the sub's amp. There's no way to reduce one without reducing the other. Placing a cap between opamp and 1M pot would break the necessary current path from opamp non-inv input to ground, causing the opamp to loose its DC stability and its output will simply peg against the supply rail. If you inserted such a cap and then added another resistor from opamp input to ground, the opamp would become happy again and then you'd have another RC lowpass filter, which would reduce the turn-on pulse as well as roll off some low freq. audio. By adjusting the value of the first coupling cap and adding a second cap+resistor after the 1M pot, you've basically created a 2 pole RC filter. So, by selecting suitable values for both, you can have a bit steeper lowpass roll-off, which might keep useable audio and still somewhat reduce the pulse. Delayed sub amp mute would be better than that, in my opinion. I think you've got a successful circuit for one already, don't you?

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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Wed 30, 2019 6:54 pm 
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.22uF is way larger than necessary. I'd try a .033uF or .02uF, if I did the RC math right. If the turn-on is still too strong then try adding another cap (.02uF?) and 1M resistor after the pot. When that fails, add delay circuit!

edit: I lowered starting cap values

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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Wed 30, 2019 8:22 pm 
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With what I calculated for minimal phase shift at 40Hz (know it may not be audible, but it is more personal preference) I would need a .068uF cap minimum.

Now the 10uF cap between the 4.7K resistor and inverting input is it absolutely necessary?

Am thinking that perhaps that cap could be contributing to the problem somewhat given when the .22uF cap charges up the momentary dc pulse would travel through to that cap and that cap has its own charge current as well.

I know the output of the buffer is 0Vdc and I am thinking the output of the filter is 0Vdc, but I don't know what adding a 20K variable in series with a 4.7K resistor with no cap will do to the DC operating point of the inverting amp.

The main reason I have the 10uF cap is so that if the filter OP-AMP fails to one rail it won't cause a catastrophic failure given the inverting amp and transistors won't be taken to one rail voltage.

Now what I could do is alter the .68uF cap to cause the same roll off as the .003uF cap does then remove the .003uF cap replacing the 470K resistor with a 500K trimpot then remove the 20K trimpot with the 500K pot thusfore now controlling the gain. That would definitely reduce the DC pulse and the phase wouldn't much matter then given the cap is now performing the job of a filter.

I do have a delay circuit that I used in my Lafayette LA-375 amp, but I'd have to see if Electronics Goldmine has any more then I'd have to replace the caps in it along with the resistors given those relays are older.

EDIT:

Now if I take the 1.1 meg control resistance and use a .001uF cap I get

fc = 144.68631190172[Hz]

Which is closer to the two pole filter

fc = 132.62911924325[Hz]

So perhaps I'll try that first.

Another thought I had.

With the amp grounded to the radio chassis that puts the full B+ across the filter cap.

What if I doubled the coupling cap value and put another of the same between the amp ground and the B+ end of the 10K resistor?

That way the only voltage on the cap will be the DC that is across the 10K resistor.

That said safety might be an issue though given if the cap were to short it would put full B+ on the amp chassis.


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Wed 30, 2019 11:23 pm 
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Quote:
Now the 10uF cap between the 4.7K resistor and inverting input is it absolutely necessary?
Probably not, but you might see a change of DC offset on the speaker output if you remove it. That's because the last 741 stage would then no longer be the sole determinant of the DC offset on the speaker. Any DC error from your buffer and/or filter will be amplified by however much gain the last stage has. The 10uF cap blocks that. Your two added opamps would contribute some error but I'm not sure it'd be a problem since your buffer and filter stages are low gain, thus lower error. I highly doubt the 10uF's presence is increasing your turn-on pulse from the radio. You can jumper across it to test.

Quote:
The main reason I have the 10uF cap is so that if the filter OP-AMP fails to one rail it won't cause a catastrophic failure given the inverting amp and transistors won't be taken to one rail voltage.
I see your concern. What do you think will happen if one of those output transistors fails? Output devices fail regularly, I've replaced quite a few in my lifetime. Many amps at least include some sort of protection circuit that opens a relay if DC appears on an output. I don't see any protection in your schematic. Properly sized fast blow fuse on the speaker will hopefully save it.

Quote:
With the amp grounded to the radio chassis that puts the full B+ across the filter cap.
What if I doubled the coupling cap value and put another of the same between the amp ground and the B+ end of the 10K resistor?
That way the only voltage on the cap will be the DC that is across the 10K resistor..
What filter cap? You must mean the coupling cap? I think you should keep the radio and amp ground tied together! Consider placing a pair of back-back zeners across the 1M pot, with zener voltage higher than any audio peak that is normal during operation. As a side benefit, they may take clip off some of the whammy of the turn-on pulse.

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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Thu 31, 2019 1:52 am 
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Yes meant coupling cap.

The amp has a fuse in each of the transformer leads going to the bridge rectifier, but I did not show them on the schematic.

They were originally 4 amps each, but at max output I only get 1.1 amps into a 4 ohm load at 40Hz so I replaced them with 2 amp normal blow fuses. What is interesting is I had the output leads short together while testing yesterday at full output and other than blowing a fuse all was ok.

I've ran this amp several hours each time I did something to it with a 4 ohm load at full output before distortion at 40Hz as I wanted to be sure the amp could handle whatever I threw at it in normal use without having a meltdown.

I'm notorious for testing the max undistorted output an amp can do as it is a good indicator of whether the amp is good or not.

If for instance it is a HI-FI amp and cannot withstand its rated output into its rated load at 20Hz for an hour or more without having a meltdown then it is no good to me.

Interestingly enough with the 10uF cap bypassed the amp only makes one dc pulse upon the radio being turned on and it takes the output briefly to -12Vdc. With the 10uF cap in circuit it does two and takes the output first to -12Vdc then +12Vdc.

So I will try the .001uF cap and see if that reduces it enough. If not I will delay the signal to the amp.

I think that I may leave the 10uF cap in place as it isn't hurting anything by being there and does provide a good safety feature.

EDIT:

Here's how the amp is at this point.

Attachment:
Bass amp 6.png
Bass amp 6.png [ 53.51 KiB | Viewed 1060 times ]


Tried removing the .003uF cap and installing the .001uF cap to feed the 1.1 meg level control along with a .002uF cap feeding the driver tube grid.

Seems to work quite good so far.

Here's a couple pics showing the installation of the jack and cap in the radio.

Used an existing hole that was large enough for the jack to fit.

Attachment:
20190130_204142.jpg
20190130_204142.jpg [ 241.42 KiB | Viewed 1060 times ]


The terminal strip is soldered to the terminal on the cap block the driver tube grid connects to.

Attachment:
20190130_204154.jpg
20190130_204154.jpg [ 230.12 KiB | Viewed 1060 times ]


Now it looks like the bakelite block still has a .015uF cap inside which is a new film cap as I can see a lead going through the right hole.

If so that will affect the results so I may look tomorrow and remove it if present.

What I may do is just clip the lead and insulate it. That way I can easily return the radio back stock.


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Thu 31, 2019 4:29 am 
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Quote:
...installing the .001uF cap to feed the 1.1 meg level control...
I think you mean .01, which makes a 15Hz lowpass into the 1M pot. 10,000pF is .01uF
Quote:
With the 10uF cap in circuit it does two and takes the output first to -12Vdc then +12Vdc.
Oh! I didn't expect that but thinking about it now, it makes sense.
Quote:
If for instance it is a HI-FI amp and cannot withstand its rated output into its rated load at 20Hz for an hour or more without having a meltdown then it is no good to me.
Another good stress test for an amp with bjt output is continuous pink noise or sinetone at around 1/3 to 1/2 full power, which is when the most heat is dissipated by the transistors

Seems like this project is well along, thanks for the ride!

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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Thu 31, 2019 5:13 am 
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You're welcome and thanks for the help.

Yes I did goof with the capacitor as I read the value wrong.

I may try to find a .001uF cap and try it.

Also I had to add the .003uF cap back as I was getting some high frequency hiss in the speaker without it.

May take the amp back to work tomorrow and see if the output has dropped any with the .01uF cap.

If it has then I'll abandon that idea and go back to the .22uF cap in the radio.

Perhaps the issue is with the .0033uF cap and it's 102 Hz frequency. Maybe a smaller cap value will work better. Perhaps I'll make the cap only large enough to where it cuts out the hiss but doesn't affect the filter any.

EDIT:

Turns out the amp had an issue where I had signal out of the amp which was a max of 1.4 Vrms before distortion, but the invert input of the 741 showed no signal but while troubleshooting the problem went away.

I suppose the issue with the cap charge current taking the output quite high could have caused the issue.

Removed the .0033uF cap from the 741 and tried it on the input to the amp. It dropped the level at 40Hz considerably.

I checked and the dual 1.1 meg pot was actually a dual 500K pot so I disconnected one section so the impedance would be 500K.

Now here's the numbers.

Single pole filter on 741

fc = 153.92160840609[Hz]

Two pole filter

fc = 159.1549430919[Hz]

Those two are extremely close.

Now if I take the 500K pot and use a .002uF cap

fc = 159.1549430919[Hz]

An exact match

I may try that cap there just to see what happens, although I'm tempted to leave things as is.

At 40 Hz I get an output of 9.00 Vrms

At 200 Hz I get an output of 2.42 Vrms

Thinking I may leave well enough alone unless you see a reason why I should use the .002uF cap on the input and use a much smaller cap in place of the .0022uF cap just to ensure the hissing is not present.

Concerning the thump caused by the turn on and off of the radio I may get one of those sequenced power strips that turn certain outlets on first and off last.

That way the radio is on first and off last.

There has to be a better way though.

Perhaps another place I can pick off the signal going to the amp that can be made low impedance that is not at B+ potential.

Attachment:
Amp.png
Amp.png [ 56.93 KiB | Viewed 1044 times ]


Another thing I am wondering about.

What if I were to use an Edcor WSM15K/15K transformer cap coupled to where the amp is currently connected to the 10K resistor?

Given the 500K impedance on the secondary the primary impedance would be quite high as well.

Only problem I could see is perhaps the cap charging would cause a pulse to occur on the secondary of the transformer.

I can try the transformer on the amp to see what it will do and I can get a rough impedance based on what coupling cap causes no drop in signal at 40Hz.

I could even use the center tap of the secondary to further reduce the audio voltage.


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Thu 31, 2019 3:18 pm 
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Filter points look fine to me, I'd leave them be as they are now. And, so you can sleep better, know that different break points can actually be a good thing when unwanted filter characteristics are not overlapped and do not add to each other. It is total performance that matters most.

The last 741 is in a "summing" config, where feedback from the output and your sub signal are mixed together into the inverting input. Depending on gain of the opamp, you may have to look really close to see any audio signal on the actual inverting input because most of it is cancelled immediately by the feedback signal.

edit: I don't think a .001uF coupling cap is what you want. It would introduce a bass rolloff at 150Hz and below. Your 10000pF (.01uF) cap is a good choice.

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Last edited by richfair on Jan Thu 31, 2019 11:18 pm, edited 1 time in total.

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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Jan Thu 31, 2019 3:40 pm 
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LOL I like a dummy was using the high pass filter calculator.

The one pole filter

fc = 153.92160840609[Hz]

The two pole filter

fc = 159.1549430919[Hz]

So because of that I cannot use the series capacitor to form a low pass. Thing is I knew that, but my mind got backwards in dealing with this amp.

The .01uF cap with the 500K pot forms a high pass

fc = 31.830988618379[Hz]

So I may leave it in place.

Here's the updated schematic and hopefully the final one.

Attachment:
Bass amp 7.png
Bass amp 7.png [ 54.32 KiB | Viewed 1039 times ]


The back to back zeners does seem like a great idea.

I need to measure the max output of the radio's amp so I will know the max level I will see across the 10K resistor as that will determine what zeners I use.

What I decided to do is use the other section of the 500K dual pot as a fixed 500K resistor in series with the amp input. I then adjusted the 20K pot to compensate for the reduced gain.

Also I had the output speaker wires touch again while the amp was at max output. The two 2 amp fuses blew again. Popped in new fuses and all is well.


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Feb Fri 01, 2019 3:55 pm 
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Well as much as I like this solid state amp I just cannot figure out any good way to eliminate the thump completely.

So I may have to go with tube amplification.

I'm thinking a 20 watt tube amp would work fine.

Either push pull such as a 6L6 amp or push pull parallel such as a quad of 6V6 like Magnavox would sometimes do.

I would then need a low pass filter added. Perhaps I could do one like Motorola did with their three channel consoles.

This wouldn't be something I would build as that would cost more than buying and restoring an amp and may not turn out as good.


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Feb Fri 01, 2019 4:39 pm 
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This mute idea will need refinement and may be more complicated than you want to mess with...
Mute turns on when sub amp is turned on (voltage above 4 volts appears on mosfet gate from amp's supply through divider). A 555 one-shot creates delay, powered somehow by the radio so it begins timing at radio turn on. Use the 555 logic output that begins at HI level (hopefully fast enough to be ahead of the turn-on pulse) and switches to LOW after a period. Connect to the mosfet gate. It will sink current from amp supply divider to near-zero. Weakness of design idea is the unpowered 555 circuit loads down the 47k zener voltage divider before the radio is turned on, you want to be sure the loading does not turn on mosfet. May need to adjust 47k value to overcome load but keep 555 current sink at comfortable value when unmuted. EDIT: Updated image, used zener in voltage divider

BTW, back to back diodes to limit voltage pulse on audio input seems like a very prudent measure, to guarantee not exceeding opamp maximums. I just assumed +-5v p-p, you should fix value based on your actual audio


Attachments:
mute fragment for tuberadio.JPG
mute fragment for tuberadio.JPG [ 49.23 KiB | Viewed 1026 times ]

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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Feb Fri 01, 2019 6:11 pm 
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That is one option, but would require another jack on the radio for the mute signal.

For now I will use the amp and just unplug it before the radio is turned off and plug it in after the radio is turned on.

Or I may install a SPST toggle switch that shorts the signal to ground after the 500K series resistor.

Now I do have a Bogen CHS-35A amp which I restored and eliminated the output transformer given it can drive a 4 ohm load directly and modified it to where the input is fed straight to the main level control.

Perhaps that amp could be used. Would need to make a low pass filter for it though.


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Feb Mon 04, 2019 6:50 am 
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I have a Magnavox push pull quad 6V6 amp on the way that is minus the output transformer which I can source from Edcor.

Will be restoring and using that amp.

Not sure if it uses a driver transformer or tubes for the phase inversion.

Will need a two pole filter that is tube based though


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Feb Tue 05, 2019 4:21 am 
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A couple of "Old School" passive RC filters in series follow by makeup gain? The stages interact but with only 2 filters it shouldn't be too hard. Or perhaps a cathode follower in a circuit that looks very much like the opamp version you just built? I have no experience with the latter. A search returned this document which looks like good reading even if the author is obsessed with bipolar power supplies on his tubes. http://www.tubecad.com/articles_2001/Tube-Based_Crossovers/Tube-Based_Crossovers.pdf

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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Feb Tue 05, 2019 12:32 pm 
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Think I'll start a separate topic about the restoration of this amp and continue there as this could get quite confusing otherwise.


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 Post subject: Re: Low pass filter addition to amplifier
PostPosted: Apr Tue 30, 2019 9:43 pm 
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I'm thinking about revisiting this amp to make it usable while I wait for the time and money to restore the tube amp.

I see one way to reduce the issue I am having.

That is to install a resistor from the + input of the buffer to ground of maybe 5-10 meg or larger if a larger value will work and using a small value capacitor between the input level control and + input of the buffer.

That way the charging current of the cap will be much smaller and as such would cause less of a problem in the amp.

I've updated the schematic to show the added resistor and cap along with the 500K series resistor I used to try and reduce the problem.

Attachment:
Bass amp 8.png
Bass amp 8.png [ 43.6 KiB | Viewed 645 times ]


Attachment:
Frequency.png
Frequency.png [ 20.7 KiB | Viewed 644 times ]


Now would those values serve to keep the momentary charging current low enough so as not to affect the circuit or will it still affect it due to the higher impedance?

EDIT:

I tried the cap and resistor.

It indeed helped the problem a great deal, but now I need to figure out how to shield the area where the resistor and cap are as hum pickup is a problem.

I can turn the level control full CCW touch the end of the cap connected to the 10Meg resistor and get a healthy buzz in the speaker.


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