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Old 19th April 2003, 11:13 PM   #11
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Default GAINCLONE PRECEDED BY A TUBE PREAMP.

Hi,

Steve,

Could it be that you misread the original question in post #1?

From what I've read so far this gainclone has no gain, well it has but it's rendered useless by the voltage divider at the input ...dunno why it is like this but since I understand it has an input impedance of 20K, why can't it be used with a tube preamp of low Zo?

I am by no means a semi-conductor expert but are al the previous posts not beside the point?

I also wonder about the input sensitivity of that gainclone.

What did I miss?

Cheers,
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Old 20th April 2003, 01:36 AM   #12
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Default Re: GAINCLONE PRECEDED BY A TUBE PREAMP.

Quote:
Originally posted by fdegrove
Hi,
From what I've read so far this gainclone has no gain, well it has but it's rendered useless by the voltage divider at the input ...dunno why it is like this but since I understand it has an input impedance of 20K, why can't it be used with a tube preamp of low Zo?

Any tube preamp with low Zo could be used with Gainclone, I think. Why not?
What I would like to know is which project I should choose among the thousands over the internet. I'm new to tube stuffs.
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Old 20th April 2003, 02:55 AM   #13
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Default Re: GAINCLONE PRECEDED BY A TUBE PREAMP.

Quote:
Originally posted by fdegrove
Could it be that you misread the original question in post #1?
Dunno. I took him to mean what he said. He wanted only his preamp to handle the voltage gain with the power amp functioning as a unity gain current buffer.

That scenario is exactly what that MOSFET buffer was designed for. But this implementation of the 3875 only makes it SEEM as if that's what's going on.

Quote:
From what I've read so far this gainclone has no gain, well it has but it's rendered useless by the voltage divider at the input ...dunno why it is like this...
It's like this because the 3875 can't be used as a unity gain buffer. So smoke and mirrors are used so that the output voltage equals the input voltage making it only seem as if the 3875 is operating at unity gain.

This may be a viable solution in such cases where your input voltages may be too high, but in this instance, it makes no sense whatsoever to attenuate the signal by 20dB only to amplify it by 20dB.

Quote:
...but since I understand it has an input impedance of 20K, why can't it be used with a tube preamp of low Zo?
It can.

Quote:
I am by no means a semi-conductor expert but are al the previous posts not beside the point?
Not really. He stated that his ultimate intent was to have a preamp being responsible for ALL of the voltage gain with the amplifier handling only current gain. If that truly is his intended goal, I don't see that pointing out that he's not really achieving that goal with the present solution as being beside the point.

He's perfectly free to stick to the present solution. I just thought it would be helpful to explain to him what's really going on so he can make that decision.

Quote:
I also wonder about the input sensitivity of that gainclone.
That will depend on his power supply voltage.

Quote:
What did I miss?
I don't know. What did you miss?

se
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Old 20th April 2003, 03:16 AM   #14
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Default Re: Re: GAINCLONE PRECEDED BY A TUBE PREAMP.

Quote:
Originally posted by Steve Eddy


He stated that his ultimate intent was to have a preamp being responsible for ALL of the voltage gain with the amplifier handling only current gain.

Why would he need two packages, if he could have everything handled by one?
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Old 20th April 2003, 03:21 AM   #15
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TI has some power operational amplifiers that are unity gain stable, some like them better than the ML series, some don't, you might want to give them a try if that's what you're looking for. But if you were to go with a single-ended class A MOSFET source follower, you'd be well off indeed
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Old 20th April 2003, 03:28 AM   #16
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Default Re: Re: Re: GAINCLONE PRECEDED BY A TUBE PREAMP.

Quote:
Originally posted by Peter Daniel
Why would he need two packages, if he could have everything handled by one?
Sure. He could just make a Gainclone integrated and be done with it as far as gain goes. But he seems to want to add some tubes into the mix.

Bottom line he seemed to know what it was he was after. And when someone already knows what they want, I don't try to convince them otherwise. In this case, I was simply explaining to him that what he's got doesn't seem to be what he's actually going for.

se
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Old 20th April 2003, 03:35 AM   #17
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I guess you explained it good enough, since he seems to change his mind already

Quote:
Originally posted by JAZZ2250


I would not think about the preamp with gainclone because most of the power op-amps for gainclone make good quality sound by themselves. If you still want to use a preamp, I would recommend those from Texas Instruments such as OPA134.

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Old 20th April 2003, 03:37 AM   #18
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Default Re: Re: Re: GAINCLONE PRECEDED BY A TUBE PREAMP.

Quote:
Originally posted by Peter Daniel

Why would he need two packages, if he could have everything handled by one?

Hahaha. Yes, you're right, Peter. I could have everything in a single package.

The main reason for two packages is that I want to make several tube preamps and to feel thier own taste. For that, I needed a tube-like or neutral power amp. I heard that some people say that gainclone (LM3875) sounds like a tube amp and some others say that it's neutral. So, that's why I chose unity (well... apparently) gain gainclone. I plan to build a mosfet power follower in the future (Thanks SE for your kind recommendation and instructions). But, not tight now.
Why not tube power amp? I can't afford to buy quality output transformers. I want to start making simple (and hopefully, not so expensive) tube preamps, first.
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Old 20th April 2003, 03:37 AM   #19
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Quote:
Originally posted by JoeBob
TI has some power operational amplifiers that are unity gain stable, some like them better than the ML series, some don't, you might want to give them a try if that's what you're looking for. But if you were to go with a single-ended class A MOSFET source follower, you'd be well off indeed
Thanks.

I haven't had any need to run a power opamp at unity gain (rather needless complexity when all you need is a current buffer) so never looked into what's out there that is unity gain stable.

The MOSFET power buffer seems more in keeping with the stated goals of his project. Though it will be far less efficient and require a lot more heatsinking than a power opamp solution.

se
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Old 20th April 2003, 03:41 AM   #20
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To evaluate tube line stages, I don't think it really matters how much gain the amp stage has, it will have it's own sonic signature anyway. The point is that LM3875 based gainclone sonic signature is easy enough to live with and some people say it's much less than what tube amps bring in.
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