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Preamp-Amp Coupling Caps
Preamp-Amp Coupling Caps
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Old 1st October 2018, 02:01 PM   #1
nebulix is offline nebulix  Canada
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Default Preamp-Amp Coupling Caps

In the attached schematic of my vintage preamp stage, I intend to replace the indicated capacitors on either side of the tone circuit with films. Currently they're 10uF electrolytics.

DF96 previously suggested to me not to go much below 1uF on these caps, for a rolloff of about 5Hz.

The preamp will be feeding a pair of LM3886 chip amps downstream. My design calls for 4.7uF film caps on the inputs.

I'm wondering if I need 4.7uF caps in the preamp as well to match the gainclones. Is there any point having 4.7uF on the amps if the preamp caps are say, only 2uF or 3.3uF? I'm going for maximum bass here.

Any suggestions would be greatly appreciated.
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File Type: jpg NC341 Preamp Caps.jpg (132.1 KB, 312 views)
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Old 1st October 2018, 02:14 PM   #2
DF96 is offline DF96  England
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The effect of a capacitor on frequency response depends on the resistors around it, so you cannot directly compare 4.7uF at one point in a circuit with 4.7uF elsewhere.

In the case of the tone control the relevant resistance is around 25k (half the pot value).
For your amp it would be whatever the input resistance is i.e. the resistor to ground after the input cap, together with whatever else is in parallel with it. You must already know what this is, because you would have needed to know to calculate the 4.7uF value you have chosen there.
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Old 1st October 2018, 02:18 PM   #3
Mooly is offline Mooly  United Kingdom
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Preamp-Amp Coupling Caps
Dropping the originals from 12.5uF to 1uF sounds pretty drastic to me and can only reduce LF response and increase phase shift.

By exactly how much isn't something I could give a quick answer to and you would need to look at how it was influenced as the pots were moved from the nominal flat position. 10 or 20uF electrolytics really are the best parts for the job here.

A 4.7uF input coupling cap on its own doesn't tell much because you need to know the impedance it works into. You should look to minimise LF attenuation in the GC (by suitable choice of coupling cap taking into account what it is working into) and also by suitable choice of feedback return cap.

Set your final wanted low frequency point by a well defined time constant at the preamp input and don't allow the various following stages to compromise that would be my advice.
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Old 2nd October 2018, 10:18 AM   #4
DF96 is offline DF96  England
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The originals are electrolytics, so are bigger than necessary in order to reduce distortion. He was asking about changing them for film caps, which would be huge. I said that if film caps were used they could be smaller values.

In any case, this is vintage audio not modern 'high end'. If it was me I would leave it as it is.
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Old 2nd October 2018, 06:48 PM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by Mooly View Post
Dropping the originals from 12.5uF to 1uF sounds pretty drastic to me and can only reduce LF response and increase phase shift.
...
In THIS (antique) circuit, C4 and C8 are the two ends of a "unity gain" network. If they are equal, there is "no" rolloff until the Q2 stage runs out of gain. (With rising bass THD before that.)

The other constraint is that C4 C8 should be much bigger than the EQ caps. Seeing 0.068u, the proposed 1u is 15X and should cause little effect.

And back up. This is an experimental "upgrade". What's the worst can happen? Fire? Explosion? Maybe lifted PCB traces. Try it and see.
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Old 2nd October 2018, 07:39 PM   #6
Mooly is offline Mooly  United Kingdom
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Preamp-Amp Coupling Caps
Lets try it I already had a discrete tone stage set up and so its wasn't much work to get from that to this.

This shows modest bass boost set with the pot, first with 12.5uF and then 1uF. I'll post the .asc in case anyone wants a play, and also the original I modded.
Attached Images
File Type: jpg T1.JPG (283.1 KB, 237 views)
File Type: jpg T2.JPG (282.0 KB, 215 views)
Attached Files
File Type: asc Vintage Discrete Tone Control.asc (3.4 KB, 3 views)
File Type: asc Discrete Tone Control.asc (5.6 KB, 3 views)
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Old 3rd October 2018, 04:45 AM   #7
nebulix is offline nebulix  Canada
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You guys are amazing. Thanks so much for your efforts to help me.

Indeed, it's a vintage audio experiment: a very simple line-level front end to the gainclone, but you'd be amazed how sweet they sound together. The preamp tempers the digital source with a bit of warmth.

This little preamp is very clean, and has nice quality pots (a far cry from today's digital controls). It's the preamp section from a turntable combo. It came with a Dual 1210 deck with a Noresco NC-341 amplifier built into the plinth.

I was going to chuck it out, but the front panel miraculously fits my gainclone 19" case with little modification, and it looks pretty -- somewhat of a canard, and quite a conversation piece. So I'd like to do what I can to make it slightly "high-ender".

I'm afraid I'm not that familiar with SPICE. Silly question: is the solid green the rolloff and the dotted is the freq. response?

I acquired some 2.2uF polypropelene caps that are just slightly larger than the original axial electrolytics. I guess I could just plug those into LTspice using your .asc files?

This whole thing is a memory-laner for me: We had the Dual 1210 TT at home when I was growing up. I now have that for my lacquer-stacker, as well as a 1225, 1218, 1219, 1249 and 601.

Interestingly, there is no RIAA eq in the circuit. They just ran the crystal cartridge straight into a line-level input. I'll only be running it with a DAC in any case.

Thanks again.
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Old 3rd October 2018, 06:36 AM   #8
Mooly is offline Mooly  United Kingdom
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Preamp-Amp Coupling Caps
The solid line is the response and shows the effect of having the bass pot set at 10k/40k. So not full boost. The dotted line is phase.

Yes, you can alter anything you want in the simulation and see the effect.
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Old 3rd October 2018, 09:30 AM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by nebulix
So I'd like to do what I can to make it slightly "high-ender".
Swapping capacitors may give you a warm glow, but it won't change the sound quality which is largely fixed by the circuit. All you will do is reduce any resale value.
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Old 3rd October 2018, 04:39 PM   #10
nebulix is offline nebulix  Canada
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Indeed. I have mixed feelings about all that, as expressed in an interview I did with Australia Sound+Image magazine last year. It's regarding a restoration I performed on a pair of Marantz Model 1 tube preamps. You may read it here:
McAllister featured in magazine Down Under | The Techs-Mechs Blog
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