Gain of the Thorsten/Peter Daniels gainclone?

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needtubes said:
Actually, with 10k and 220k, the gain is 23... I don't know howhigh I would push the gain, though. (personally, for me, g=23 was higher than I like... but it worked out ok)

Hi,

Actually, gain is 22. For inverting topology gain is R2/R1. Maximum gain is limited with bandwith you need.

Regards
 

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moamps said:


Hi,

Actually, gain is 22. For inverting topology gain is R2/R1. Maximum gain is limited with bandwith you need.

Regards

If you account for the potentiometer in the gainclone, which is in series with R1, then it is not always 22, and changes with the movement of the wiper on the potentiometer.

I am wondering if the actual perfect gain for the gainclone is quite a bit lower then 22, since the average listening level isn't full volume. It might be interesting to figure out what exactly the gain is at the setting that you use on the gainclone in inverting configuration, and setting the same volume level and same gain in non-inverting configuration to compare.

Assuming a 100k potentiometer (thorsten's schematic), and the desired listening level is in the middle (linearly in the middle), then the gain would be 220k/(10k+50k) = a gain of 3.667.

--
Brian
 
Konnichiwa,

BrianGT said:


Assuming a 100k potentiometer (thorsten's schematic), and the desired listening level is in the middle (linearly in the middle), then the gain would be 220k/(10k+50k) = a gain of 3.667.


Your calculation is based on assumption (middle of travel) and incomplete math. The reality looks a lot more complex due to a number of interactions.

Sayonara
 
Kuei Yang Wang said:
Konnichiwa,



Your calculation is based on assumption (middle of travel) and incomplete math. The reality looks a lot more complex due to a number of interactions.

Sayonara

Incomplete maths indeed - at mid position of pot... a quick check on the ol' sliderule... trumpets blaring... 6.286 .

The 100K pot must be regarded as 25K as the assumption must be that the source impedance, say from your CD Player, is low. So:

(220)/(10+25) = 6.286 .

When wiper is at the top (hold on to your ears) the gain = 22. So likewise when at the bottom, except gain of 22 times zero (no signal to amplify).

Now the math are complete! :nod:

Seriously though, anyone got a diff result?

Isn't it interesting that the calculation is different from shunt feedback, where gain is (R1+R2)/R2, whereas series (inverted) feedback is R1/R2.

Joe R.

PS: Oh yes, my preferred gain for LM3875 is +33dB.
 
Konnichiwa,

Joe Rasmussen said:


Incomplete maths indeed - at mid position of pot... a quick check on the ol' sliderule... trumpets blaring... 6.286 .

Seriously though, anyone got a diff result?


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
 
Kuei Yang Wang said:
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?
 
Kuei Yang Wang said:

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.:dodgy:
 
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Joined 2003
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.
 
Kuei Yang Wang said:
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.
 
UrSv said:


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.

Joe R.
 
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