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  #1  
Old 13th October 2016, 05:13 PM
bikerhifinut bikerhifinut is offline
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Default cathode bypass capacitors in pre3, omitting them?

I have been musing over this circuit since obtaining a set of parts to build one.

As designed the circuit has a gain of around 15.5dB as I understand it.

So I wondered how much gain would be lost by omitting one, or both, cathode bypass capacitors on the ECC82. I think if my theory is right that this introduces an amount of cathode feedback which cuts gain, without upsetting the DC operating conditions.

Am I barking up a dead end alley here?

I even figured that leaving out a stage of amplification on the pre3 might give the same results, its a zero feedback design as far as I can see so there shouldn't be any complications there.

My power amps are all around 500mV or less sensitivity so unity gain is usually enough, with the odd source I have only outputting 200 mV so a gain of 6db would be plenty in that case. I had a notion to include a simple switch to isolate the bypass cap(s) in order to have adjustable gain.

And just to add complication, I was also pondering the use of something like a 6SN7 instead of ECC82, just because I can but am willing to be persuaded otherwise.

Please no shunt pot suggestions, my feelings on that mod are well known to most of the regulars here. And I feel it treats the symptoms rather than the problem so to speak. I have the desire to use my switched attenuator in this as my set up in the new house allows the preamp and sources close to the listening position with power amp(s) at the far end of the room and very short speaker wires.
A lot of ideas all at once, apologies for that.
Thanks in advance gentlemen.

Crazy Andy.
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Old 19th October 2016, 01:19 PM
John Caswell John Caswell is offline
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Default Re: cathode bypass capacitors in pre3, omitting them?

Hi Andy,
I can't see any reason why you cannot do this, not sure how much it will reduce the gain, I suggest you try it and see. Unfortunately my Pre3 is too buried in the system to quickly try it out.
What did do with mine is to fit an intermediate preset volume control 250KΩ -1MΩ in place of L1-L2 and R2 (1MΩ) and set it to a sensible listening level, you could then remove the pot measure it and replace it with two fixed resistors. This does have the advantage of reducing the noise from the first stage.
I also tried removing the first stage completely but that made the whole thing sound pretty lifeless.
When time allows I am going to experiment with feedback a) to see if it is possible and b) to see what effect/it sounds like, but these things are on the back burner at present due to serious family problems.

John
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Old 19th October 2016, 01:38 PM
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pre65 pre65 is offline
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Default Re: cathode bypass capacitors in pre3, omitting them?

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Originally Posted by John Caswell View Post
but these things are on the back burner at present due to serious family problems.

John
Sorry to hear that John, I hope all will be well before too long.
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Everything in this post is my honest opinion based on what i thought I knew at that very moment in time.
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Old 19th October 2016, 02:13 PM
bikerhifinut bikerhifinut is offline
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Default Re: cathode bypass capacitors in pre3, omitting them?

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Sorry to hear that John, I hope all will be well before too long.
Me too, John.

I'm investigating these mods too so if I find anything I'll post what I discover.
interesting about the first stage removal as I pondered that too.
I have a sneaky suspicion that removing a bypass cap might impair the bass end? But as its an easy experiment it's worth doing.

I'm now thinking of putting the phono3 board in the same box as I have a large case. (I may buy a new PCB off Matthew for this).

Andy
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Old 19th October 2016, 03:45 PM
John Caswell John Caswell is offline
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Default Re: cathode bypass capacitors in pre3, omitting them?

Hi all,
Thank you for your comments, much appreciated.
Andy, (I'm now thinking of putting the phono3 board in the same box as I have a large case. (I may buy a new PCB off Matthew for this)) not a bad idea, Pre3 case is a bit cramped. You should have seen it before I attacked the layout way back when.
My Pre3 is in a 2U case with RF remote volume, relay input switching and a few other bells and whistles.
It enables you to play with the both the SUTs and the Pre3 optxs to minimise hum.

John
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Old 21st October 2016, 09:08 PM
FreddieT FreddieT is offline
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Default Re: cathode bypass capacitors in pre3, omitting them?

John,

I hope all turns out well for you and yours. You may remember a while back that you soldered the bypass caps back in a Pre3 that I borrowed from Matthew and (with permission) removed the caps. I can't remember why I didn't do it myself. I used to have a good memory I think.

Andy,

I do remember that the gain was reduced (as theory suggests, you are introducing negative feedback) but can't find my notes (see above) on my measurements. The bass was affected, but it should be less attenuated (the bypass cap sets the LF roll off) relative to the midband. However, the Pre3 caps are quite large and I don't remember significant differences. Incidentally smaller values are often used as treble boosters in guitar amps. Give it a go, and I'll continue to search on and off for my scribblings.

F.
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