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Old 3rd March 2009, 12:53 AM   #1
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Default Parafeed with NP electrolytic?

Has anyone tried making a parafeed amp using a non-polarized electrolytic for the parafeed cap? This is for a guitar amp so I am not concerned about distortion. I assume it will work as long as the cap is rated for twice B+ and there is some NFB for stability? Can it be pushed so far as to use roll-your-own NP's (2 'lytics back-to-back with protection diodes)? TIA
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Old 3rd March 2009, 01:26 AM   #2
Gordy is offline Gordy  United Kingdom
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Presume you mean between anode and o/p transformer.

I would have assumed voltage rating only needed to exceed anode voltage + max signal swing.

No need for nfb or anything else as far as a simple parafeed cap is concerned. (Although you might need nfb for other aspects of the amp I supose).

If the inductance of the o/p transformer is highish then cap capacitance value need not be great; just a few micro-Farads. Watch out for ripple-current rating though.

Roll-your-own + diodes?!?!? I have no idea... but why bother when off-the-shelf items are cheap-as-chips!

Good luck.
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Old 3rd March 2009, 01:46 AM   #3
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Excuse me, but why don't you use an ordinary polarized electrolytic cap?
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Old 3rd March 2009, 01:58 AM   #4
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Quote:
Originally posted by Gordy
I would have assumed voltage rating only needed to exceed anode voltage + max signal swing.
Which is twice B+ for an SE amp AFAIK(?)

Quote:
Originally posted by Gordy
No need for nfb or anything else as far as a simple parafeed cap is concerned.
Really? Even with electrolytics as nonlinear as they are?

Quote:
Originally posted by Gordy
Roll-your-own + diodes?!?!? I have no idea... but why bother when off-the-shelf items are cheap-as-chips!
I can't easily find NP's rated over 100V. Where are they?

Quote:
Originally posted by Wavebourn
Excuse me, but why don't you use an ordinary polarized electrolytic cap?
I wasn't sure that this would work as all the hifi parafeed projects use film caps. Where does the + go?
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Old 3rd March 2009, 02:09 AM   #5
twystd is offline twystd  United States
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I wouldn't recommend it. I've done some radical recaps (all caps) of guitar amps, using some very high quality caps (no electrolytics). All the players were very pleased with the results.

Guitar amps are a different animal than audio amps. You purposely build in clipping and distortion, so you'd think you could use lesser quality components in them. A bud (he designs custom amps both audio and guitar) and I were discussing this the other day. The gist of the discussion was, anything that sounds good in an audio amp, sounds good in a guitar amp.

The quality of a parafeed cap is VERY important to the tone of a parafeed amp. I'm afraid a crappy cap will sound like a crappy cap in a guitar or audio amp, ESPECIALLY as a parafeed cap.

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Old 3rd March 2009, 02:12 AM   #6
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Quote:
Originally posted by leadbelly
I wasn't sure that this would work as all the hifi parafeed projects use film caps. Where does the + go?
On the positive side ...

The hifi amps use film caps as they sound better, and at the point that you have put $1000 in to your amp, a $20 film cap instead of a $2 electrolytic is incidental.

Generally, in parafeed, one side of the cap connects to the plate and the other to the transformer primary with the other side of the primary connected to ground or the cathode. Thus, the + side connects to the plate.

You'll also see the transformer connected to the plate with the cap between the other primary lead and either ground or the cathode. In this case, the - side connects to the cathode or ground.
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Old 3rd March 2009, 02:16 AM   #7
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Quote:
Originally posted by twystd
The gist of the discussion was, anything that sounds good in an audio amp, sounds good in a guitar amp.
It is too general of a statement to be correct. Stiff regulated power supples sound good in hifi amps and are a good way to ruin your guitar amp. Same goes for 300B's. And subwoofers. And ribbon tweeters.

Quote:
Originally posted by twystd
The quality of a parafeed cap is VERY important to the tone of a parafeed amp. I'm afraid a crappy cap will sound like a crappy cap in a guitar or audio amp, ESPECIALLY as a parafeed cap.
It is no more or less in the signal path than the power supply caps and will have the same influence.
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Old 3rd March 2009, 02:24 AM   #8
Gordy is offline Gordy  United Kingdom
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Quote:
Originally posted by leadbelly
Which is twice B+ for an SE amp AFAIK(?)
If you use an inductor as the anode load and drive it to the max then yes. (I had been thinking of a constant-current-source load for my own amp and hence overlooked inductor loads when answering your post!)


Quote:
Originally posted by leadbelly
Really? Even with electrolytics as nonlinear as they are?

I wasn't sure that this would work as all the hifi parafeed projects use film caps. Where does the + go?
(As Doug said: normally + goes to anode, - to o/p transformer primary).
Polarised caps have often been used at amp outputs and seem fine when they have a polarising voltage across them.


Quote:
Originally posted by leadbelly
I can't easily find NP's rated over 100V. Where are they?
As Mr. Wavebourne said, just use a polarised one. On the other hand, if you do go with a film cap check that the ripple current rating is OK, as it is a power amp after all.
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Old 3rd March 2009, 03:04 AM   #9
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Just use a film cap... we are tyalking less than 10 uF (we have 3.3 uF on our parafeed amps)

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Old 3rd March 2009, 03:07 AM   #10
twystd is offline twystd  United States
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Quote:
Originally posted by dsavitsk


It is too general of a statement to be correct. Stiff regulated power supples sound good in hifi amps and are a good way to ruin your guitar amp. Same goes for 300B's. And subwoofers. And ribbon tweeters.
Perhaps I should have made myself clearer, I meant components normally found in guitar amps like caps, tubes. Nobody would seriously entertain ribbon tweeters, sub woofers. 300Bs very debateable, if driven into clipping by the preceding stage might sound pretty good.


Quote:
Originally posted by dsavitsk

It is no more or less in the signal path than the power supply caps and will have the same influence.

The parafeed cap is very much more in the signal path than the power supply caps. The parafeed cap goes either between the plate and transformer primary, or the transformer primary and ground, therefore directly in the signal path. In a parafeed amp, the power supply cap is somewhat isolated from the signal coming from the plate by the plate choke, or even more so if you use a CCS instead of a choke.

Some feel that parafeed isolates the power supply, and the power supply caps are not important. I don't agree with that from direct experience with trying different caps on the bread board, and hearing a distinct difference, but less so than in conventional single ended designs. In the case of conventional single ended designs I do agree the last power supply cap is directly in the signal path.

Besides, IMO electrolytics don't sound good in power supplies, and I don't use them there either. As a matter of fact I avoid electrolytics whenever possible. Of course everybody has an opinion and you're certainly welcome to yours.

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