I have built couple of solid state amplifiers. Recently, I'm much into the sound of so-called Gainclone which is based on LM3875 (National Semiconductors Power OP-Amp). Many people said that its sound was pretty much close to that of tubes. I don't know if it's true or not because I have no experience with tube stuffs. So, I decided to make a tube preamp to mate with LM3875.
The tube preamp is supposed to do all the voltage amplification and power amp based on LM3875 will do the current amplification. Once I thought to build Foreplay, but I changed my mind to go with a complete DIY. I would like to get some recommendation from the tube guys here which project would be good for my plan. So, please feel free post what you guys think. Gain of 20-25 dB should be high enough for me. Thanks.
The tube preamp is supposed to do all the voltage amplification and power amp based on LM3875 will do the current amplification. Once I thought to build Foreplay, but I changed my mind to go with a complete DIY. I would like to get some recommendation from the tube guys here which project would be good for my plan. So, please feel free post what you guys think. Gain of 20-25 dB should be high enough for me. Thanks.
It works.
Thanks for your comment. But...
It works, actually, without any problem.
I already built a Gainclone with unity gain.
Please check the following link if you're interested.
Click here
Now, what about the tube preamp?
Any recommendation, please?
Steve Eddy said:
Thanks for your comment. But...
It works, actually, without any problem.
I already built a Gainclone with unity gain.
Please check the following link if you're interested.
Click here
Now, what about the tube preamp?
Any recommendation, please?
Re: It works.
I read it. And the LM-3875 isn't operating at unity gain. It's operating at a gain of about 11. In other words, it's applying voltage gain. It SEEMS as if it's operating at unity gain because your circuit is attenuating the input signal by the same factor as the voltage gain of the 3875. That's not the same thing as if the 3875 were operating at unity gain. With the 3875 operating with a voltage gain of 10 and attenuating the input signal by the same amount, all you're doing is increasing the signal-to-noise ratio.
If that works for you, that's great. But it's not doing what you indicated you want it to do (i.e. the preamp handing voltage gain with the power amp only applying current gain). Both your preamp and your power amp are applying voltage gain. You've just got a resistive attenuator between the two that makes it only seem that that's the case.
It's kind of like how Congress only made it seem like we had a budget surplus.
If you want the real deal, you'd use a follower.
se
JAZZ2250 said:
I read it. And the LM-3875 isn't operating at unity gain. It's operating at a gain of about 11. In other words, it's applying voltage gain. It SEEMS as if it's operating at unity gain because your circuit is attenuating the input signal by the same factor as the voltage gain of the 3875. That's not the same thing as if the 3875 were operating at unity gain. With the 3875 operating with a voltage gain of 10 and attenuating the input signal by the same amount, all you're doing is increasing the signal-to-noise ratio.
If that works for you, that's great. But it's not doing what you indicated you want it to do (i.e. the preamp handing voltage gain with the power amp only applying current gain). Both your preamp and your power amp are applying voltage gain. You've just got a resistive attenuator between the two that makes it only seem that that's the case.
It's kind of like how Congress only made it seem like we had a budget surplus.
If you want the real deal, you'd use a follower.
se
analog_sa said:
Apparently he's using this circuit:
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<img src="http://www.diyaudio.com/forums/attachment.php?s=&postid=112202">
</center>
Although the input resistor is the same value as the feedback resistor (220k), it's actually in parallel with the 22k shunt resistor at the inverting input. So you have 220k||22k, which comes to 20k. So the voltage gain is 220k/20k which comes to 11 (about 21dB)
And that 220k/22k combination also forms a 1:10 voltage divider so that the input signal is attenuated by a factor of 10, or 21dB.
But the fact remains that the LM-3875 is functioning <b>EXACTLY</b> as if you were to remove the 22k resistor and replaced the 220k input resistor with a 20k resistor.
Again, it only SEEMS as if it's operating at unity gain because the signal's being attenuated by the same amount as the voltage gain. You've got 21dB of signal attenuation combined with 21dB of voltage gain.
Smoke and mirrors.
mc2 laid it all out very nicely in this post here:
http://www.diyaudio.com/forums/showthread.php?postid=129587#post129587
se
Re: Re: It works.
So, are you saying that I should use LM3875 in a follower configuration? Would it be stable without oscillation? It seems my quqestion here should be posted in the solid state forum. But, I started here anyway, so I'll just keep going here.
And, I'm still waiting for a tube preamp recommendation. Please...
Steve Eddy said:
So, are you saying that I should use LM3875 in a follower configuration? Would it be stable without oscillation? It seems my quqestion here should be posted in the solid state forum. But, I started here anyway, so I'll just keep going here.
And, I'm still waiting for a tube preamp recommendation. Please...
Re: Re: Re: It works.
You can't. It's an opamp. There is no "follower configuration" per se.
A follower is basically just a tube or BJT or FET configured as a common-anode/collector/drain amplifier. What are also referred to as cathode/emitter/source followers. Their output voltage follows the input voltage with no voltage gain (in the real world, the voltage gain is going to be slightly less than 1).
An example of a follower (in this case a source follower) is that MOSFET power follower circuit I referenced you to earlier.
At best all you can do with an opamp is configure it as a unity gain buffer. But the LM-3875 isn't stable at unity gain. That's why you're using that circuit you're using now, so it's voltage gain remains at about 10 or above.
If you're happy with what you have now, then by all means stick with it. I'm not saying there's anything wrong with it. I'm simply pointing out that it's not giving you that preamp for voltage gain only/power amp for current gain only configuration you said you wanted.
But if that's what you'd rather have, then you'll either need to find a power opamp that's unity gain stable, or use a follower circuit.
Fine with me 'til the moderators say otherwise.
'Fraid I can't be of much help to you there. Tubes aren't my forte.
se
JAZZ2250 said:
You can't. It's an opamp. There is no "follower configuration" per se.
A follower is basically just a tube or BJT or FET configured as a common-anode/collector/drain amplifier. What are also referred to as cathode/emitter/source followers. Their output voltage follows the input voltage with no voltage gain (in the real world, the voltage gain is going to be slightly less than 1).
An example of a follower (in this case a source follower) is that MOSFET power follower circuit I referenced you to earlier.
At best all you can do with an opamp is configure it as a unity gain buffer. But the LM-3875 isn't stable at unity gain. That's why you're using that circuit you're using now, so it's voltage gain remains at about 10 or above.
If you're happy with what you have now, then by all means stick with it. I'm not saying there's anything wrong with it. I'm simply pointing out that it's not giving you that preamp for voltage gain only/power amp for current gain only configuration you said you wanted.
But if that's what you'd rather have, then you'll either need to find a power opamp that's unity gain stable, or use a follower circuit.
It seems my quqestion here should be posted in the solid state forum. But, I started here anyway, so I'll just keep going here.
Fine with me 'til the moderators say otherwise.
And, I'm still waiting for a tube preamp recommendation. Please...
'Fraid I can't be of much help to you there. Tubes aren't my forte.
se
analog_sa said:
Great. Unfortunately JAZZ2250 hasn't found the LM-3875 to be stable at unity gain.
It doesn't. It relies on a reasonably competent designer to read the data sheets and take the appropriate measures to accommodate the range of source impedance values the circuit's likely to encounter.
se
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,
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,
Re: GAINCLONE PRECEDED BY A TUBE PREAMP.
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.
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.
It can.
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.
That will depend on his power supply voltage.
I don't know. What did you miss?
se
fdegrove said:
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.
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.
It can.
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.
That will depend on his power supply voltage.
I don't know. What did you miss?
se
Re: Re: Re: GAINCLONE PRECEDED BY A TUBE PREAMP.
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
Peter Daniel said: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
I guess you explained it good enough, since he seems to change his mind already
JAZZ2250 said:
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.
Re: Re: Re: GAINCLONE PRECEDED BY A TUBE PREAMP.
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.
Peter Daniel said:
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.
JoeBob said: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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