In all the inverted gainclones schematics I saw, the input impedance is quite low. For instance when a 10k pot is used for volume. I believe Thorsten recommends a 22k resistor in the minus input.
As I want to keep my tube preamp, which cures the sound from the CDP, I am looking for a quite higher input impedance, something like 200k to put things optimal. As I dont need the volume pot, a simple resistor will do the trick. Anyone tried this?
Miguel
As I want to keep my tube preamp, which cures the sound from the CDP, I am looking for a quite higher input impedance, something like 200k to put things optimal. As I dont need the volume pot, a simple resistor will do the trick. Anyone tried this?
Miguel
You can always try it, it's only two resistors ! But I would not recommend it. I noticed a big improvement when going from 100 k to 50 k. But I changed more things at the same time (eg. better switch, other resistors).
I think you get too much noise etc. from a 200 k resistor. Besides, the input impedance will still be low because of the 10 k that goes to the - input (virtual ground).
Fedde
miguel2 said:
Your tube preamp can't drive anything lower than a 200k load? That'd require a 4.4 meg feedback resistor. Them's some pretty high impedances which will leave you much more susceptible to interference. Sure you can't get by with something more reasonable like say, 50k?
Anyway, it'll be just fine without the pot. And since your tube preamp likely has output coupling caps, you can eliminate the cap as well.
se
fedde said:
If you're not going to use the pot (its only purpose is for attenuation) then you don't replace it with a resistor, you just leave it.
se
My pre is now driving a ZenV4, and it is confortable (although I can't get the maximum power from the Zen), so it can drive lower impedances. But, as I said, I was thinking in an optimal solution.
Fedde, why is the - input a virtual ground? I believe the current to the - input will be very low.
But I will try some values there. BTW I am using OPA541s. If things get worse I do have a pair of OPA134 on hand, just in case. But then I would not have a gainclone
Miguel
Fedde, why is the - input a virtual ground? I believe the current to the - input will be very low.
But I will try some values there. BTW I am using OPA541s. If things get worse I do have a pair of OPA134 on hand, just in case. But then I would not have a gainclone
Miguel
The + input of the amp is at zero. So the - input is also approx. at zero (due to the feedback and the very high amplification factor of the opamp). This means that the 10 k resistor from the pot to the - input is paralled with the pot resistance. So the total input resistance with a 50 k pot varies between 50 k//10 k and 50+10 k, so between 8.3 k and 60 k.
Fedde
Fedde
Input impedance...
The maximal input impedance is 50k...not 60k...
The maximal input impedance is when the volume is at 0...the wiper of the pot is at ground...and the preamp only "see" the 50K resistence of the pot...
regards
The maximal input impedance is 50k...not 60k...
The maximal input impedance is when the volume is at 0...the wiper of the pot is at ground...and the preamp only "see" the 50K resistence of the pot...
regards
miguel2 said:
Miguel,
Hehe… EF86? I tried it with inverted gainclone. But was not satisfied. No, definitely, not a solution for gainclone. Btw, EF86 can drive some 70-80kOhm loads.
If you omit the pot, you should form inverting input’s path to ground.
Pedja
Pedja,
You remember well 🙂
It is working good on the Zen so I will give it a try with the gainclone. You know, I have two 9 pin sockets left on the preamp, waiting, waiting
. I just cant find another E88CC (on this junk store I know).
Miguel
You remember well 🙂
It is working good on the Zen so I will give it a try with the gainclone. You know, I have two 9 pin sockets left on the preamp, waiting, waiting
. I just cant find another E88CC (on this junk store I know).Miguel
Re: Input impedance...
I just wanted to correct my mistake, but you were before me... 🙁
Fedde
Tube_Dude said:
I just wanted to correct my mistake, but you were before me... 🙁
Fedde
Of course I remember
There are less than 10 people using this tube in the whole world! Ok, maybe more, but still less than 100. Usage of that tube is one of the rare ways to improve the sound of my other power amp (comparing to the option with passive pot), but unfortunately it is not the case with the inverted (so-called) gainclone.
Of course, I’d be glad to hear your experience on this.
Pedja
There are less than 10 people using this tube in the whole world! Ok, maybe more, but still less than 100. Usage of that tube is one of the rare ways to improve the sound of my other power amp (comparing to the option with passive pot), but unfortunately it is not the case with the inverted (so-called) gainclone.
Of course, I’d be glad to hear your experience on this.
Pedja
Ok, got one channel up and running. Isn't it great when you just fire it up and it makes music🙂 ? I used a crappy PSU I have, single supply transformer splited in a dual supply by resistors, with a virtual ground, but it made some nice noises.
So I though of using my integrated rotel PSU. What I have in mind is to connect some wires from the transformer or after the bridge and run them to the gainclone. The rotel keeps working (I hope).
But I had no success on it. The transformer has 4 leads out: a neutral yellow, a 21VAC red, a neutral black and a 21 VAC red, by this order. I connected the + and - of the bridge rectifier to the gainclone. Then I connected the black neutral wire to the earth of the gainclone. First I heard some big hum and when I put music on the fuse of the rotel blown. Should I have used the yellow neutral?
The two red wires and the yellow have turrets before the fuses on the rotel. The black has a turret between the capacitors. I can't see the circuit without taking the whole thing apart...
Miguel
So I though of using my integrated rotel PSU. What I have in mind is to connect some wires from the transformer or after the bridge and run them to the gainclone. The rotel keeps working (I hope).
But I had no success on it. The transformer has 4 leads out: a neutral yellow, a 21VAC red, a neutral black and a 21 VAC red, by this order. I connected the + and - of the bridge rectifier to the gainclone. Then I connected the black neutral wire to the earth of the gainclone. First I heard some big hum and when I put music on the fuse of the rotel blown. Should I have used the yellow neutral?
The two red wires and the yellow have turrets before the fuses on the rotel. The black has a turret between the capacitors. I can't see the circuit without taking the whole thing apart...
Miguel
Hmmmm... yesterday I measured my GC with my scope. It seems to have some oscillation (since the latest mods !?). I see a 8 Mhz sine with 10 mV amplitude. My scope is not really reliable or correctly calibrated. So the amplitude could well be different...
Luckily I have some plans to reduce this problem. A problem could be the + input of the opamp connected very close to the power supply star ground. This could result in unwanted supply/input->output interaction. In the past that problem was not a big issue with the resistor (18 k or earlier 220 k). So I'll try to lead the +input to the stepped att...
Fedde
Luckily I have some plans to reduce this problem. A problem could be the + input of the opamp connected very close to the power supply star ground. This could result in unwanted supply/input->output interaction. In the past that problem was not a big issue with the resistor (18 k or earlier 220 k). So I'll try to lead the +input to the stepped att...
Fedde
i got the first channel of my gainclone working today.
even though i have wires everwhere (big mess) and i am using a rubbish speaker, the sound quality is amasing.
i cant wait to finish the other channel and take some pics.
[edit]: what fuse should i use? my power supply has a fuse in the mains pluig, fuses on the back and 1A current limit but i want to be safe. 🙄 should i use a fast or a slow-blow fuse? what current rating should i use?
even though i have wires everwhere (big mess) and i am using a rubbish speaker, the sound quality is amasing.
i cant wait to finish the other channel and take some pics.
[edit]: what fuse should i use? my power supply has a fuse in the mains pluig, fuses on the back and 1A current limit but i want to be safe. 🙄 should i use a fast or a slow-blow fuse? what current rating should i use?
Matt, I use a 1.6AT on the primaries side of my PSU's and that should be sufficient. Depends how many more of the sound degrading little so and so's you want to put in!
I'm a GC picture fan so get them up ASAP
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