Hello,
As the title says, I need a buffer/gain stage for my 8 channel LM3886 amp which till now was driven by a behringer CX3400 active crossover.
My amp is based on Linkwitz Riley desing at http://www.linkwitzlab.com/images/graphics/3886amp.gif
This amp has an input sensitivity of 1,1V.
Recently I replace behringer with a miniDSP 2x4 and although I am very happy with the sound of miniDSP I have a amplification problem since miniDSP 2x4 has an output of 0.9V max.
My problem is that miniDSP output (0,9V) cannot drive the amp to full power and this is very annoying especially with low level recording even with the volume at full the sound is not loud enough for me. I assume there is also an additional voltage loss from the long interconnects (2 meter) that I have to use since the amp and minidsp are not located close to each other.
My thoughts are to amplify miniDSP’s output to 6db or more by adding a buffer/ gain stage which is supposed to be beneficial for gainclones either way.
So, I would like your suggestions in order to add a 4 channel buffer/gain stage between these two units.
My thoughts are:
A) To add an opamp buffer/gain stage with a 6db gain as the first stage described at ESP preamp project 88.
B) Discrete buffers as described at Decibel Dungeon pages the problem with this is I don’t know if they can modified to provide a 6 db gain or not.
C) Not the best but probably the easiest to by two readymade preamps like this from Ebay and remove the pots.
I forgot to mention that I am using miniDSP’s digital volume control, and I already have an adjustable symmetrical psu with maximum output of +-14V that I would like to use for this project.
The buffer/ gain stage is going to be placed inside the miniDSP box.
Thanks in advance.
As the title says, I need a buffer/gain stage for my 8 channel LM3886 amp which till now was driven by a behringer CX3400 active crossover.
My amp is based on Linkwitz Riley desing at http://www.linkwitzlab.com/images/graphics/3886amp.gif
This amp has an input sensitivity of 1,1V.
Recently I replace behringer with a miniDSP 2x4 and although I am very happy with the sound of miniDSP I have a amplification problem since miniDSP 2x4 has an output of 0.9V max.
My problem is that miniDSP output (0,9V) cannot drive the amp to full power and this is very annoying especially with low level recording even with the volume at full the sound is not loud enough for me. I assume there is also an additional voltage loss from the long interconnects (2 meter) that I have to use since the amp and minidsp are not located close to each other.
My thoughts are to amplify miniDSP’s output to 6db or more by adding a buffer/ gain stage which is supposed to be beneficial for gainclones either way.
So, I would like your suggestions in order to add a 4 channel buffer/gain stage between these two units.
My thoughts are:
A) To add an opamp buffer/gain stage with a 6db gain as the first stage described at ESP preamp project 88.
B) Discrete buffers as described at Decibel Dungeon pages the problem with this is I don’t know if they can modified to provide a 6 db gain or not.
C) Not the best but probably the easiest to by two readymade preamps like this from Ebay and remove the pots.
I forgot to mention that I am using miniDSP’s digital volume control, and I already have an adjustable symmetrical psu with maximum output of +-14V that I would like to use for this project.
The buffer/ gain stage is going to be placed inside the miniDSP box.
Thanks in advance.
Nothing wrong with an opamp gain stage. It's just the buffer with two feedback resistors. It really is the classic text book implementation.
An alternative idea. Modify the LM3886 stage. Reducing the 1K feedback return would increase the gain.
An alternative idea. Modify the LM3886 stage. Reducing the 1K feedback return would increase the gain.
A 3886 has a maximum gain of 10times (+20dB). But at this gain the phase margin is quite low.
The 3886 performs better at reproducing the input signal when the gain is increased to about 28 to 30times. (+28dB to +30dB).
The 3886 will still work well with the gain up at 40 times (+32dB).
Only when more gain than +32dB is required would an extra gain stage be worth fitting.
That extra gain stage could be in the Pre-Amp. There it could be made switchable, so that higher level sources could pass straight through and thus avoid excess gain + excess attenuation.
A non-inverting 3886 does not need a buffer or preconditioning of the signal.
An inverting 3886 does benefit from a stage that preconditions the line to allow quality signal into the low impedance presented by the inverting input.
I would prefer to put that pre-conditioning stage back at the source (the Pre-Amp) and call it a low impedance buffer stage.
The 3886 performs better at reproducing the input signal when the gain is increased to about 28 to 30times. (+28dB to +30dB).
The 3886 will still work well with the gain up at 40 times (+32dB).
Only when more gain than +32dB is required would an extra gain stage be worth fitting.
That extra gain stage could be in the Pre-Amp. There it could be made switchable, so that higher level sources could pass straight through and thus avoid excess gain + excess attenuation.
A non-inverting 3886 does not need a buffer or preconditioning of the signal.
An inverting 3886 does benefit from a stage that preconditions the line to allow quality signal into the low impedance presented by the inverting input.
I would prefer to put that pre-conditioning stage back at the source (the Pre-Amp) and call it a low impedance buffer stage.
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Thanks for your fast response.
I would prefer to build a buffer instead of modifying gainclone's gain stage because i am afraid that with the long interconnects i use and low level recordings miniDSP will still be incapable to fully drive the amp.
I would prefer to build a buffer instead of modifying gainclone's gain stage because i am afraid that with the long interconnects i use and low level recordings miniDSP will still be incapable to fully drive the amp.
What about the 3 transistor buffer, described at Decibel Dungeon pages.
Can this stage modified to provide amplification of 6 db or more?
Can this stage modified to provide amplification of 6 db or more?
Well a buffer is really easy... but you want a gain stage 🙂
I can probably find lots more worked examples but this was first out of the bag that I remembered,
http://www.diyaudio.com/forums/analog-line-level/167495-simple-stereo-gain-stage.html#post2198757
read the whole thread though.
I can probably find lots more worked examples but this was first out of the bag that I remembered,
http://www.diyaudio.com/forums/analog-line-level/167495-simple-stereo-gain-stage.html#post2198757
read the whole thread though.
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If you want discrete then something like this is excellent although originally designed for single rail,
http://www.diyaudio.com/forums/anal...reamp-only-4-bjt-transistors.html#post3101459
tbh though you will not really better an opamp stage using something like the OPA2134 dual opamp.
http://www.diyaudio.com/forums/anal...reamp-only-4-bjt-transistors.html#post3101459
tbh though you will not really better an opamp stage using something like the OPA2134 dual opamp.
I guess you have wright, I had good experiences in the past with opamps like OPA2134, LM4562 & LT1028, why not now.
Discrete stages could wait for the moment.
Thanks.
Discrete stages could wait for the moment.
Thanks.
Thinking again the option to increase the gain of the amp and according to my calculations the current gain of the amp is 20,6.
So if i replace the feedback resistor of 1k with a 687 ohm, the gain will be increase to 29,5 which i think will be enough.
Is that correct?
So if i replace the feedback resistor of 1k with a 687 ohm, the gain will be increase to 29,5 which i think will be enough.
Is that correct?
That's correct. The voltage gain is 29.5 with the new value.
On a practical level don't get hung up on exact resistor values for gain setting. 687 ohm or 620 or 560. The difference in perceived level won't be as great as the numbers suggests.
Also from a theory point. You have attenuator in the 1K and 19.6K input resistors. Again in actual use its a non issue.
Best way is to try values around the 680 ohm mark and take it from there.
Have you got Ci fitted that is on your circuit ?
On a practical level don't get hung up on exact resistor values for gain setting. 687 ohm or 620 or 560. The difference in perceived level won't be as great as the numbers suggests.
Also from a theory point. You have attenuator in the 1K and 19.6K input resistors. Again in actual use its a non issue.
Best way is to try values around the 680 ohm mark and take it from there.
Have you got Ci fitted that is on your circuit ?
Yes Ci is fitted.
Beside of changing the gain of the amp, do i have the option to change Rin from 1K to 0,5 in order to increase the input sensitivity?
Altering the 1K Rin won't make any audible difference. Levels have to change a lot for our ears to notice, far more than tweaking that 1K will make.
Again on practical matters...
Ci will probably need to be increased in value to compensate for the lower value of the feedback resistor. Unless you have something oversized in there anyway. Not doing so will cause the bass response to roll off that bit earlier. So doubling the value would make sense.
Again on practical matters...
Ci will probably need to be increased in value to compensate for the lower value of the feedback resistor. Unless you have something oversized in there anyway. Not doing so will cause the bass response to roll off that bit earlier. So doubling the value would make sense.
if i remember correctly, i believe i used the values suggested by the chematic.
ci = 110 μf non polar for the woofer channels.
ci= 10 μf for the tweeter channels.
what values you suggest?
ci = 110 μf non polar for the woofer channels.
ci= 10 μf for the tweeter channels.
what values you suggest?
I like the MooseFet preamp from Classic Valve Design, available as just a board, or with some parts. I have IRF510 fets on mine.
Go to the next preferred values, so that's 220uf (two 470uf's to make bi-polar) and 22uf (or two 47uf for bi-polar).
One more question,
how important is the quality of the caps at this position and what caps do you suggest me to use?
how important is the quality of the caps at this position and what caps do you suggest me to use?
CI (NFB cap) is a cap at the inverting input and that is exactly as important as any other input cap. This is a good spot to test drive a couple of different options and to try a tiny bypass cap all for the purpose of finding out what you like best.
Standard commercial/industrial grade electrolytics works well in this location.
But if you make the electrolytic value too low, then it can have avoidable voltage across it and that voltage gives rise to avoidable distortions.
Make the cap big enough and you don't need fancy priced capacitors.
But if you make the electrolytic value too low, then it can have avoidable voltage across it and that voltage gives rise to avoidable distortions.
Make the cap big enough and you don't need fancy priced capacitors.
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