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 Post subject: Buffer amp (problem)
PostPosted: Jan Tue 05, 2016 6:43 pm 
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I am looking to build a buffer amp that can handle the output of my McIntosh C24 preamp's center channel (maximum 10 volts) and drive a 25K load.

I would prefer it be unity gain if possible.

I figure an op amp or transistor will work.

I can use whatever B+ is needed.

I do have a bluetooth receiver and a DAC that both require 5 volts so I would be looking at incorporating a 5Vdc regulated supply with the power supply for the buffer.

What do I need for the buffer circuit?


Last edited by Tube Radio on Feb Fri 26, 2016 1:34 am, edited 1 time in total.

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 Post subject: Re: Buffer amp
PostPosted: Jan Tue 05, 2016 9:14 pm 
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Need to build a cathode follower or with transistors, emitter follower. No gain but good buffer.

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 Post subject: Re: Buffer amp
PostPosted: Jan Tue 05, 2016 9:17 pm 
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To insure unity gain, an op-amp circuit would be best. But, there are a few questions that need answered before a design can be made:

1. While I would assume that the 10 volt max preamp output signal level is RMS, we all know what 'assume' means. So I must ask is it RMS, peak, or peak-to-peak? This is needed to determine the supply voltage(s) required.

2. What is the output impedance of the C24 preamp center channel?

3. Of secondary importance, but nice to know, what is the frequency response required.

John

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 Post subject: Re: Buffer amp
PostPosted: Jan Tue 05, 2016 9:19 pm 
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Tons of unity gain stable opamps out there too. The TL07x series is a good one with its high-impedance JFET input and almost junk bin price.

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 Post subject: Re: Buffer amp
PostPosted: Jan Tue 05, 2016 10:50 pm 
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I'm assuming it is 10 Vrms.

Not sure of the output impedance (I tried to find it, but only could find the specs for the preamp's input impedance), but it uses a .47uF cap as the output coupling capacitor and lists a frequency response of 20-20KHz and based on the frequency response spec +0 to -.5db from 20 Hz to 20KHz and an online high pass filter calculator which uses the output coupling cap value and a resistor value to figure the high pass frequency, I got 100K.

Frequency response would need to be at least 20-100Hz as this will drive a low pass crossover for a powered subwoofer.

Would be nice if I could also incorporate a 24 db/octave low pass filter in the buffer so I don't need a separate piece of equipment for that function. The crossover frequency would need to be 70Hz.


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 Post subject: Re: Buffer amp
PostPosted: Jan Tue 05, 2016 11:51 pm 
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Sounds like a great job for a quad opamp. Could even tune the unit as a whole with variable resistors to attain unity gain after the filters.

Is this stereo or mono?

I'm building a tube pre-amp for my 3 channel system that will have a stereo PP 6AQ5 setup for highs and mids and a single Eico HF-20 subwoofer amplifier. So far I'm planning to use an initial buffer which goes into the stereo crossover with recovery amps after the highs and into a mixer/recovery amp for the combined lows.

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 Post subject: Re: Buffer amp
PostPosted: Jan Wed 06, 2016 3:11 am 
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This will be mono.

Now if it is possible to build a HP and LP crossover without it affecting the sound going to the Lafayette, then I would build a stereo high pass and mono low pass.

For the Lafayette, the audio is being input right after the volume control (volume control disconnected) top bypass the input and tone control stages. The speakers use a 6 db/octave crossover.

In order to keep everything in phase I would need a 18 db/octave high pass for the Lafayette, right?

If so would it be possible to use it right before the Lafayette without any amplification?


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 Post subject: Re: Buffer amp
PostPosted: Jan Wed 06, 2016 3:39 am 
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Tube Radio wrote:
In order to keep everything in phase I would need a 18 db/octave high pass for the Lafayette, right?


Gonna be honest, that went a bit over my head. :?:

But, as long as you are only doing recovery amplification i don't see why input level couldn't equal output level after the filters.

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 Post subject: Re: Buffer amp
PostPosted: Jan Wed 06, 2016 4:08 am 
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For the Lafayette I had to use a series resistor to drop gain some with a resistor to ground after the series resistor.

I might add the volume control back in and feed the signal in at the control itself then I'll add the filter before the control.

I'm thinking there will be enough gain after the added filter given how much I had to reduce the gain initially.

Would cascading two of these work for the low pass?

http://www.electronics-tutorials.ws/fil ... gif?81223b

The whole thing is here

http://www.electronics-tutorials.ws/fil ... ter_5.html

I also found this which looks like a good read.

http://www.ti.com/lit/ml/sloa088/sloa088.pdf

I do like the simple low pass filter using four op amps though.

That is easy for me to calculate.

The other designs getting 12 db/octave with one op amp are harder to calculate as I would need to do the math by hand.

EDIT:

Using the online low pass filter calculator I can use a 100K resistor and a .022uF capacitor which will get me fc=72.34Hz

I can build four of those filters and use a quad op amp set up for unity gain.

Concerning the op amp which one would be recommended and what supply voltage should I use and should it be a + and - supply.

I can do the same for the high pass only reversing the caps and resistors to form the high pass.

Now given the speakers use a 6 db/octave crossover that would put them 90 degrees out of phase with the sub so I figured a three pole filter for the high pass would put the two in phase, but would also mean the hp crossover would be at 18 db/octave versus 24 db/octave for the low pass.

That may cause its own problems so I may just build the high pass at 24 db/octave then look for some speakers that have 12 db/octave crossovers provided the sub doesn't integrate well with the main speakers.


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 Post subject: Re: Buffer amp
PostPosted: Jan Wed 06, 2016 9:35 pm 
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Note: This reply is based on your initial request and may not be what you want overall. Below is a schematic for the buffer amp with 24dB per octave rolloff at 70Hz:

Attachment:
70Hz 4-Pole LPF Schem Sm.jpg
70Hz 4-Pole LPF Schem Sm.jpg [ 27.28 KiB | Viewed 3231 times ]


And here is the frequency response curve (phase is the dotted plot):

Attachment:
70Hz 4-Pole LPF Response Sm.jpg
70Hz 4-Pole LPF Response Sm.jpg [ 47.35 KiB | Viewed 3231 times ]


U2 and U3 form the four pole Sallen-Key Butterworth filter while U1 buffers the input. U1 is necessary because the Sallen-Key filter requires a low (ideally zero) driving impedance or at least one that is definitely known (so it can be accounted for). The Sallen-Key topology was chosen because it inherently provides unity gain in the passband.

R5 and R6 provide a DC return path for the non-inverting input of U1. R6 also provides a discharge path for C5 and together they set the low frequency rolloff at about 6Hz at the input. R5 also provides some measure of input protection for U1.

C6 provides a low frequency rolloff in combination to the stated value of load resistance simulated by R7. Combined with the input rolloff, the overall low end -3dB frequency is about 9Hz.

As to the choice of op-amp, the stated maximum input voltage is 10VRMS or 28.3V, P-P. Allowing for the maximum output swing within 2 volts of the supply rails, an op-amp with no less than 32 volts total supply voltage is needed. I should also have a high input impedance with low bias and offset currents so as not to interfere with the reasonable filter component values and to keep offset voltages low. Finally, the op-amp must have a decent gain-bandwidth and be capable of driving the specified load. The Texas Instruments TLE2064 quad op-amp easily fills the bill. http://www.ti.com/lit/ds/symlink/tle2064.pdf

John

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 Post subject: Re: Buffer amp
PostPosted: Jan Wed 06, 2016 11:25 pm 
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Thanks so much. That will work great.

Would a conventional power supply work or will I need a + and - supply?

Also would this same circuit work for a high pass if I swap the capacitors and resistors in the filters?


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 Post subject: Re: Buffer amp
PostPosted: Jan Thu 07, 2016 2:24 am 
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Yes, you'll need a +/-18v power supply to handle the 10VRMS signal with headroom to prevent clipping.

And, yes, by swapping the resistors and capacitors you'll have a high pass filter.
Attachment:
High Pass Filter Sections.jpg
High Pass Filter Sections.jpg [ 47.08 KiB | Viewed 3222 times ]


The procedure to find the values for Butterworth filters is:

1. Set both capacitors to the same value (C1 = C2 = C).

2. Look up the values of ai and bi for the filter order in the table below. Each additional order adds 6dB per octave. That's why the low pass filter needed a fourth order filter (6dB x 4 = 24dB).

Attachment:
Butterworth Filter Coefs.jpg
Butterworth Filter Coefs.jpg [ 29.68 KiB | Viewed 3222 times ]


3. Pick a convenient value for C.

4. For each second order filter section calculate R1 and R2: R1 = 1/(Pi x fc x C x ai); R2 = 1/(4 x Pi x fc x C x bi)

Note: If the filter requirements call for an odd order filter use the equation for R1 above with ai = a1 from the table in the circuit for the first order filter and combine that with the required number of second order filters. For example, a fifth order filter would have one first order section followed by two second order sections.

John

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 Post subject: Re: Buffer amp
PostPosted: Jan Thu 07, 2016 3:34 pm 
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Since the preamp has output coupling caps do I need the input coupling cap in the circuit or can I do without it?

also can I just use the same values for the components in the low pass filter or do I need to figure new component values for the high pass filter?

If I need new component values I'll need a bit of help calculating them as I am not very good with math.

EDIT:

Looks like this power transformer will work http://www.antekinc.com/as-0518-50va-18v-transformer/

I can use a bridge rectifier to derive the + and - voltages with the center tap of the transformer grounded.

I will then use a 7818 and 7918 to provide the regulated + and - 18Vdc

My plan for the 5 volts is to use a 7812 then install a 12 volt automotive power jack and just plug in one of those dual USB chargers. That would be the easiest way to do it.

I might use a DC-DC converter (pin compatible to a 7812) so I can have more than 1 amp of available current. Then I could use a three USB jack charger so I can have one to charge a cellphone or MP3 player if so desired.

I will more than likely use a small ammo case for this project as they are readily available around where I live, provide full shielding and are easy to work with.

Am thinking of using 1/4" jacks for the stereo inputs and outputs and 1/4" mono jacks for the center channel input and output.

That said if I am going to build this as stereo why not combine the stereo channels to mono inside the crossover?

Not exactly sure how to accomplish that in the proper way.


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 Post subject: Re: Buffer amp
PostPosted: Jan Thu 07, 2016 9:35 pm 
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No, you don't need an input coupling cap as long as the preamp output cap has low leakage. Leakage current would develop a voltage across the 2.2Mohm input resistor which would offset the signal from ground. If the offset is great enough, the signal could be clipped as the op-amp outputs "run into the rails". The advantage of using the input coupling cap is that you get to specify a low leakage type and won't have to worry about what is in the preamp.

As to the values for the high pass filter, no, you cannot use the same values as the two pairs of caps in the low pass filter are of different values, while in the high pass filter they are the same value, whatever that turns out to be. I can figure the values you need if you can tell me what corner frequency you need. Is it 70Hz also?

The transformer and rectifier arrangement will work, but at 50VA that transformer might be a little overkill unless you're planning on running a whole lot more circuitry than a few filters! (Nice transformer, though.)

John

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 Post subject: Re: Buffer amp
PostPosted: Jan Thu 07, 2016 10:34 pm 
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It will be 70Hz as well.

The transformer will also power a voltage regulator so I can power a bluetooth receiver and a DAC with each one requiring 5Vdc 500mA.

I'm assuming a 7812 will work for regulation to the dual USB charging adapter.

The reason for both is I had gotten the bluetooth receiver when I used a sony as a preamp and it had a digital input so rather than render that device useless I just bought a DAC to use with it.

My only concern is integrating the sub with the main speakers. They will be 90 degrees out of phase with each other.

In order to avoid a ground loop, I am thinking of using 1/4" insulated jacks.

That said I wonder if I should use an isolated dc/dc converter for the 12 Vdc?

The main reason is to eliminate the two switch mode power supplies and also to free up two plugs in the power strip.

Oh ok I'll leave the input coupling caps in then. Won't hurt anything.

Now another question.

Should I go with my plan of using the center channel output to drive the low pass crossover or should I sum the left and right channels to drive the low pass filter?


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 Post subject: Re: Buffer amp
PostPosted: Jan Fri 08, 2016 8:04 pm 
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Here is the 70Hz high pass filter:

Attachment:
70Hz 4-Pole HPF Schem Sm.jpg
70Hz 4-Pole HPF Schem Sm.jpg [ 30.16 KiB | Viewed 3166 times ]


Note that in this and the low pass filter, all filter resistors should be 1% metal film types and the filter capacitors should be 5% or better.

What will cause the sub to be 90 degrees out of phase with the main speakers?

I would combine the left and right channels to drive the low pass filter as there will be low frequency information contained in both channels.

John

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 Post subject: Re: Buffer amp
PostPosted: Jan Fri 08, 2016 8:26 pm 
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The crossover in the speakers is a 6db/octave and both the high pass and low pass filters will be 24 db/octave and there is no crossover on the sub as one is not needed there. Wouldn't that make the main speakers and sub somewhat out of phase with each other?

Thanks for the calculations.

I will update the schematic and post it here when I am done with it.

I'm thinking of maybe just building a separate supply for the 5 volt devices as it may be easier to do so.

The center channel is derived from a left and right channel mix in the preamp.

How would I combine the left and right channels in the filter itself?

Since I will not be having a 5Vdc supply will this transformer work? http://www.antekinc.com/an-0220-25va-20v-transformer/ or would this one be better http://www.antekinc.com/an-0118-10va-18v-transformer/

EDIT:

Since I will have three op amp sections left over I suppose that I could just connect the + inputs of two op amp sections together before the 47K resistors and capacitor couple the outputs of the two op amps into a couple resistors connected together at one end which will feed the low pass filter. Will that require increasing the value of the input coupling caps?


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 Post subject: Re: Buffer amp
PostPosted: Jan Sun 10, 2016 5:20 am 
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Here's the schematic using all three channels from my preamp.

Image

Here's the schematic with only two channels needed.

Image


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 Post subject: Re: Buffer amp
PostPosted: Jan Thu 21, 2016 12:43 pm 
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While at my local weekend flea market this past weekend I saw some ammo cases. I asked the seller and they were $8 each. I bought one to use for this project.

Next I'll buy the power transformer.

Concerning the actual circuitry, should I build each section on a separate pc board or just use one larger pc board.

I also plan on using sockets for the op-amps in order to make them easy to replace should one go bad.


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 Post subject: Re: Buffer amp
PostPosted: Jan Thu 21, 2016 5:12 pm 
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Hey Tube, got a question with the opamps, the TLE2064 package has 4 units, with either of the designs you are using 9 or 11 of them. What would you do with the non used opamps? ground inputs and outputs?

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