Gain of the Thorsten/Peter Daniels gainclone? - Page 2 - diyAudio
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Old 21st July 2003, 02:27 PM   #11
UrSv is offline UrSv  Sweden
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Quote:
Originally posted by Kuei Yang Wang
Konnichiwa,



Well, if you calculate the actual gain from the input of the Pot with the pot in the center....

Then you get 50K : 50K//10k = 1:7 which in turn is amplified by 22, giving a net of 3.1428, yet the noisegain is as you have pointed out 6.286.

As said, it is a complex sort of circuit (simplicity is deceptive) but it works okay, especially if the feedback resistor is increased to 330K, which should avoid the sound degradation observed with a (22k) resistor from negative input to ground.

There are a great many ways to skin this particular Dog (I never skin cat's, I love cat's - can't stand dogs though).

Sayonara
Interesting. What is the reason for the sound degradation as noted by some and how can the increase to 330 K avoid it? And what about the 18 K from pos input to ground?
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Old 21st July 2003, 02:54 PM   #12
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Quote:
Originally posted by Kuei Yang Wang

As said, it is a complex sort of circuit (simplicity is deceptive) but it works okay, especially if the feedback resistor is increased to 330K, which should avoid the sound degradation observed with a (22k) resistor from negative input to ground.
My GC is a power amp.
I have the 22k resistor to ground and I don't notice any degradation.
But I don't have any resistor to ground before de input cap.
And no pot.
It seams to me that if people use the 22k resistor to ground together with a resistor to ground before the cap they may be messing things up.
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Old 21st July 2003, 03:39 PM   #13
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my way of determining amp gain is to consider the following factors:

1) most pre-amps output 1VRMS (or 1.4Vpp).
2) if you use 40v rail, the maximum output is probably 35v rail (assuming some losses at transformer and output devices).
3) so you want to amplify the 1.4Vpp signal to 35Vpp output and the gain is 25x.

of course, you may want to factor in output power, etc. as well in a real calculation but the above will set you to the ballpark.

My favorite is actually 28x as it gets a very "scientific" 100w out of a 8ohm load.
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Old 21st July 2003, 03:52 PM   #14
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Quote:
Originally posted by millwood

My favorite is actually 28x as it gets a very "scientific" 100w out of a 8ohm load.
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Old 21st July 2003, 04:00 PM   #15
moamps is offline moamps  Croatia
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Quote:
Originally posted by millwood
[B]my way of determining amp gain is to consider the following factors:
1) most pre-amps output 1VRMS (or 1.4Vpp).
1VRMS is ca 2.8Vpp or 1.4Vp
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Old 21st July 2003, 06:04 PM   #16
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Quote:
Originally posted by carlosfm


with a 1Vrms signal, and 28x gain, the output signal swings 28Vrms, which on 8ohm speakers generates (28v)^2/8=100w.

Quote:
Originally posted by moamps


1VRMS is ca 2.8Vpp or 1.4Vp

my fault,
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Old 22nd July 2003, 09:58 AM   #17
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Quote:
Originally posted by millwood

with a 1Vrms signal, and 28x gain, the output signal swings 28Vrms, which on 8ohm speakers generates (28v)^2/8=100w.
Even using the maximum allowed PSU voltage, you can configure an LM3875 with 28x gain (or more) and you won't get 100w (I'm talking RMS).
It would be nice...
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Old 22nd July 2003, 10:31 AM   #18
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Quote:
Originally posted by carlosfm


Even using the maximum allowed PSU voltage, you can configure an LM3875 with 28x gain (or more) and you won't get 100w (I'm talking RMS).
It would be nice...

I was thinking about a normal amp, not the gainclone specifically.

Yeah, it would have been nice,
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Old 22nd July 2003, 12:16 PM   #19
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Quote:
Originally posted by Kuei Yang Wang
Konnichiwa,



Well, if you calculate the actual gain from the input of the Pot with the pot in the center....

Then you get 50K : 50K//10k = 1:7 which in turn is amplified by 22, giving a net of 3.1428, yet the noisegain is as you have pointed out 6.286.

As said, it is a complex sort of circuit (simplicity is deceptive) but it works okay, especially if the feedback resistor is increased to 330K, which should avoid the sound degradation observed with a (22k) resistor from negative input to ground.

There are a great many ways to skin this particular Dog (I never skin cat's, I love cat's - can't stand dogs though).

Sayonara
I concur, in fact I loaded up CircuitMaker and constructed the circuit, put 1V into it and got 3.14V out. Interesting thing is that varying and even deleting the 22K made no difference, still around 3.14V out. It varies the noisegain but not the voltage gain. This is still assuming mid position of 100K pot. It occurs to me that it doesn't change the actual voltage gain because the 22K is across the virtual earth (- input) and real earth, so no change as there is no signal across it - at least in theory there should be a complete nul.

But changing the 'noisegain' is an improvement in two ways that I can see, 1) better stability (enhanced further by increasing 220K to 300K) and 2) controlling DC off-set.

It struck me from early on with your first schematic, that the middle position of the 100K pot was its Achilles heal - the 20dB gain required by National to ensure stability - but that has now been corrected.

Am I reading your thinking correctly on this?

Joe R.
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Old 22nd July 2003, 12:21 PM   #20
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Quote:
Originally posted by UrSv


Interesting. What is the reason for the sound degradation as noted by some and how can the increase to 330 K avoid it? And what about the 18 K from pos input to ground?
I'd like to know that too... sound degradation. Ideally the 18K should be by-passed, does the fact it isn't create a sonic degradation?

The 18K, if I may answer this, is close in value to 220K in parallel with the 22K. This keeps the DC Off-set on the output low. The bipolar inputs draws current and if you don't balance the circuit it shows up on the output and puts DC into your speakers.

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