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 Post subject: Heathkit W-5M amp ground question
PostPosted: Jan Sun 15, 2017 8:59 am 
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I'm looking to get a set of Heathkit W-5M amps. I had a single one before that I acquired in high school and regret selling it. Anyway, each of these amps is mono, so two of them needed for stereo. However they need a pre-amp and it looks like the only choice I can find is also mono, so two of them would be needed. Which means two volume controls, two power switches, etc.

What I'm thinking of doing is using the WA-P2 schematic and build my own stereo version, which would basically be the WA-P2 times 2 in the same case. Here's the concern. If I do the standard chassis grounding on this, it would mean the 2 amps (W-5M's) would have their chassis grounds now tied together through this pre-amp. Could this cause problems? If so, would there be an issue with not having the pre-amp chassis connected to the ground?

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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Sun 15, 2017 9:08 am 
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Heathkit made a number of stereo preamps back in late 50's through the early 60's. The AA-11, AA-141/141A, and SP-2/2A come to mind.

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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Sun 15, 2017 9:37 am 
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Looking into some of those, I think my debit card just ran out into traffic. Just out of curiosity, would it hurt the amps if their grounds were connected?

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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Sun 15, 2017 1:01 pm 
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TPAairman wrote:
Just out of curiosity, would it hurt the amps if their grounds were connected?
Since those are transformer powered amplifiers there would be no problem tying their grounds together. The preamplifier should also be transformer powered to prevent a problem there.

Curtis Eickerman

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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Sun 15, 2017 6:49 pm 
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The Heathkit WA-P2 preamp gets its power for filaments and B+ directly from the power amp, and has no separate power transformer. I don't see this as an issue though. You can also build a copy of one of the later stereo preamps from any manufacturer, it doesn't necessarily have to be a Heathkit circuit that you use. You could power your preamp from the power amp, or build one with its own power supply.

If you are building your own stereo version, just tie the two amplifier chassis together with a heavy braided bonding strap and you should be good to go.

Note that often in order to reduce hum, particularly when using a magnetic phono input on these units, the whole system also has to be bonded to a good earth ground, or at least the power line ground.

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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Sun 15, 2017 7:29 pm 
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I think I may look at buying an SP-2. The panel looks like it would match the W-5M's well. Though I see a bit of overtime in my future.

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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Sun 15, 2017 9:23 pm 
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Back in 1962 I bought a book titled "High-Fidelity, design, construction, measurements" by "gernsback library no. 48". In it I found a schematic for the Heathkit WA-P1 preamp. I built two of them on the same chassis. I also built two separate Stancor versions of the Williamson power amplifiers, each with their own power supplys. In connecting these together I had no hum problems.
If anyone is interested, this book is available used from Amazon for $10.25. First published in 1953.

In this same book there is another schematic for a preamp with a 23 position switch for the volume control tying the terminals together with resistors to equal 1 Meg. Every 3 terminals they ty in a resistor and capacitor for compensation.

I used this volume setup in the WA-P1 preamps.

I still have all these chassis and one day I am going to take them down from the top shelf and restore them. They always sounded great and I think they still will after 40 years in storage.

Scotty.


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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Sun 15, 2017 11:06 pm 
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G.S.D. wrote:
In it I found a schematic for the Heathkit WA-P1 preamp. I built two of them on the same chassis.


I found the schematic, but I can't find much else. Looks pretty straight forward, but one question - what is the turnover switch for?

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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Sun 15, 2017 11:36 pm 
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The term "normal audio input" is probably debatable. For full output it is looking for .004 volts at the magnetic input and 0.2 volts at the tuner and xtal pickup input.
The turnover switch selects either 78 rpm recordings or LP recordings.

Scotty.


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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Mon 16, 2017 12:51 am 
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By "normal input" I mean something like my iPod or a CD player.

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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Mon 16, 2017 1:10 am 
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You would have to check the output of those devices and act accordingly. My guess (I'm usually wrong) is that the phono input would be good. There are people on here that know more about this than me, maybe they will chime in.

Scotty.


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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Mon 16, 2017 2:47 pm 
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TPAairman wrote:
However they need a pre-amp and it looks like the only choice I can find is also mono, so two of them would be needed. Which means two volume controls, two power switches, etc.

I ran my pair from a Hafler DH-100 solid-state stereo preamp.

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 Post subject: Re: Heathkit W-5M amp ground question
PostPosted: Jan Tue 17, 2017 2:41 am 
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As for mixing solid state and vacuum audio equipment, a word of caution.

The outputs from tube pre-amps may go through some extreme voltage swings as tubes warm up and coupling capacitors charge up. A solid-state power amp that "warms up" rather quickly may not like this or, may damage or stress speakers.

Tube pre-amps, designed to drive tube power amps, may find the input impedance of some solid-state power amps a bit lower than they like.

Charlie


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