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Old 22nd April 2005, 11:40 AM   #1
Klimon is offline Klimon  Belgium
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Default changing (de)coupling cap values

Hello,

I've just changed my amp's (cheap and crappy) input decoupling caps and coupling caps between El34's (in pp) and driver (6U8A) with paper in oil: in short: dramatical results, much clearer, lifelike sound, for the first time I can FEEL bassguitar: much better bottomend.

I found some similarly entitled threads, but there never seems to be any conclusive, general answer...so I'll put my question: is there any use in augmenting the values of these caps? From previous threads (about input caps) I've learned that some say the higher you go, the better the bottomend, but your midrange sounds less good, others recommend a higher value without reserve. If you'ld recommend higher values, how much higher? (now the input caps are 0.2mf, the coupling caps are 0.47 each (=2/channel, PP).

Another tweak I remember having read about, but can't find back: is it a good idea to remove the input decoupling cap + resistor, knowing that in my sources (cd, minidisk) there are output decoupling caps? Could I measure how much DC is present and then decide whether to remove them or not?

Thanks in advance for your much appreciated reactions!!!
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Old 22nd April 2005, 11:56 AM   #2
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You seem to liberally intermix the terms coupling and decoupling. From what i understand you have only modified coupling caps so far.

Depending upon the topology input coupling caps are generally not needed in tube designs not counting the odd fixed bias or grid leak.

AS far as values go i always choose the smallest i can get away with. Good caps are generally expensive and i see no point paying a premium for bigger values. Yes, bigger caps inherently seem to have a more 'bassy' sound but it seems like a very expensive way to tone-correct.
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Old 22nd April 2005, 01:27 PM   #3
Klimon is offline Klimon  Belgium
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Thanks for the reply, analog. I think I'll remove the output caps from my sources then. Do you know if it's possible to 'overdo' it with the amp coupling cap values? I have plenty of Philips old stock pio caps, so money is not a criterium here. I know I can experiment, but a little advice from more experienced tweakers is always a blessing + I'm already spending my days soldering - listening - soldering - listening - ad infinitum!!!

Greets!
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Old 23rd April 2005, 05:54 PM   #4
percy is offline percy  United States
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Default Re: changing (de)coupling cap values

Quote:
Originally posted by Klimon
Hello,

I've just changed my amp's (cheap and crappy) input decoupling caps and coupling caps between El34's (in pp) and driver (6U8A) with paper in oil: in short: dramatical results, much clearer, lifelike sound, for the first time I can FEEL bassguitar: much better bottomend.

I found some similarly entitled threads, but there never seems to be any conclusive, general answer...so I'll put my question: is there any use in augmenting the values of these caps? From previous threads (about input caps) I've learned that some say the higher you go, the better the bottomend, but your midrange sounds less good, others recommend a higher value without reserve. If you'ld recommend higher values, how much higher? (now the input caps are 0.2mf, the coupling caps are 0.47 each (=2/channel, PP).

Another tweak I remember having read about, but can't find back: is it a good idea to remove the input decoupling cap + resistor, knowing that in my sources (cd, minidisk) there are output decoupling caps? Could I measure how much DC is present and then decide whether to remove them or not?

Thanks in advance for your much appreciated reactions!!!

f = 1 / (2*pi*R*C)

Generally this is good estimate for the coupling caps, where R is the grid resistor of the next stage, f is lowest frequency you'd like to pass.

Beyond this point (cap value) I'd say the Quality of the capacitor matters more than the value.
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Old 23rd April 2005, 06:22 PM   #5
dhaen is offline dhaen  Europe
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Quote:
Do you know if it's possible to 'overdo' it with the amp coupling cap values?
Yes. Higher values increase the time taken to recover from overload. Sometimes this is called "blocking".
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Old 23rd April 2005, 06:38 PM   #6
Klimon is offline Klimon  Belgium
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I calculated the theoretical lowest frequency for input stage and output stage, with the standard C (and R) values... This results in 1.7hz and 3hz... I'm quite sure I didn't miscalculate, so those caps won't be blocking any low frequencies I guess... Off to the next tweak! (diy stepped attenuator and better Q+higher value power supply caps+ bypass with pio) - thanks for sharing!!!
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Old 23rd April 2005, 07:14 PM   #7
dhaen is offline dhaen  Europe
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Please understand the meaning of "blocking" in this context. It is explaind in the latter part of this thread .
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Old 23rd April 2005, 09:06 PM   #8
Klimon is offline Klimon  Belgium
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I understand your first thread; I used the word blocking in another meaning (as: not letting through), just to avoid confusion... Going further on the blocking (your blocking , it would maybe be a good idea to go the other way around and to lower the C's values; hereby not influencing the lowend (as 3hz seems ridiculously low - maybe 20 hz is enough in the real world, margin included) but reducing the negative effect caused by blocking. All nice in theory, but will I really hear a difference? (...say yes and I might try it)
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Old 23rd April 2005, 09:31 PM   #9
dhaen is offline dhaen  Europe
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Sorry to have mis-judged your understanding
Yes, 20Hz is plenty low enough for music. The output transformers in your amp will probably be failing by that frequency.
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Old 25th April 2005, 07:12 AM   #10
Klimon is offline Klimon  Belgium
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Almost there...

When removing the input coupling caps, should:
a. the resistor to ground be also removed
b. the resistor value be recalculated with the formula (supra)
c. the resistor left unchanged?

My electronic knowledge is only very basic, so I try to look at schematics from people whose ability I do not doubt, as diyparadiso.com here in Belgium (in general, he does not use input caps or resistor to ground, but there's always a resistor in series between input and cathode). Andrea Ciuffoli (audiodesignguide.com) doesn't use input caps either, nor a resistor in series before cathode, but always a resistor to ground. Different people, different styles?
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