Mark Levinson 26 pre-amp schematics - Page 2

Extreme_Boky · 12th October 2018, 11:05 AM

Very nice, thank you Prime3p14

sgrossklass · 07:38 PM

Good job. Quite impressive circuitry, the voltage regulators alone are pretty fancy. Output noise should be unspectacular but generally OK, there is only one stage of volume control by the looks of it.

Looks like it should be easy enough to implement some bootstrapped cascode goodness via a capacitor across R150-D30 (and whatever the other channel equivalent is). Something in the double-digit µF range should do just fine, I think, maybe even a bit less if that results in turn-on issues. That should bring distortion down even further.

I did spot one potential problem - when high gain mode is enabled, C55 is about an order of magnitude too small, so the amplifier would end up being on the bass-light side. (I presume it's a film cap?) Parallel some more capacitance if needed (maybe two 47-100µ back-to-back).

With some more C in that spot, there'd be a realistic chance of bringing output noise down further as well. Right now it's by no means terrible but just slightly better than average for a dedicated pre, being hampered by moderately high feedback resistor values. Here are some values to bring the wideband noise floor down by about 3 dB:
R140/141/142 = 1k0 / 154R / 4k22
and in order to avoid increasing output loading:
R2 = R193 = 11-12k, matched / <1%
(Should output loading be a noteworthy contributor to distortion, you can always move the input-side end of R193 to ground. The balanced output is still going to work, just 6 dB quieter and with some common-mode component. In the stock circuit, this would bring main amp output loading from about 3.7k to 14k. Not as much benefit when modified, of course.)

Resulting noise floor should be little higher than 3 µV, good enough for 95-ish dB/W/m speakers with +30 dB worth of power amp following. That's 90 µV on the output at 44 dB of gain or good integrated amp level. Those with super high efficiency speakers - use an attenuator between pre and power amp or a variable input or generally lower gain (e.g. 22-26 dB) power amp, with the power amp not being too noisy in itself (good ones will manage around 30 µV). Top-end pres from the late '70s to early '90s would often be fitted with two-stage volume controls, usually employing 4-gang pots.

ALRAINBOW · 01:31 AM

Hello all. I have an issue with my no 26. It was fine till I moved it now one channel is lower. .it acts the same in se and bal both input and output. At one point if I moved the psu cable into the pre amp a bit it worked fine. Now it's always like this. I just finishe'd changing all the caps in the preamp . And ty for posting the schematic. I did test the voltages at the psu seems fine. Any ideas have any ideas ?
Thanks in advance.

ALRAINBOW · 3rd February 2019, 01:29 AM

Also once it's fixed any mods are very welcome. It's powering my no 33 amps. I need it working on balancertain mode .

AVWERK · 3rd February 2019, 01:35 AM

A labor of love indeed.
I would burn out way to early !

Regards
David

ALRAINBOW · 3rd February 2019, 01:38 AM

Hahaha I just spent 6 hours taking it apart the pre and changed the caps.
Found 3 bad caps too. I used a cap tester after I removed caps.
But it still is the same lol.

ALRAINBOW · 3rd February 2019, 03:04 AM

Ok I have followed the psu from est psu to internal regulators all is ok so far.
Oddly if use the pre for a headphone amp it's sounds fine
Only when I connect it to my amps it's low on one channel
I have done all the usual stuff reversed channels and cables
I am now using a krell preamp all is ok. Any ideas ?

ALRAINBOW · 3rd February 2019, 05:24 AM

Guys a happy ending it was the Dip sws inside in doing the de cap I hit one somehow
Set to max output all balanced out now. Also it has more low level details too.
I'll print out the upgrade for the preamp and check it out.
Thanks for the schematic very useful
Wonder of I should change the large caps inside the psu case too.
Any input. ??

Prime3p14 · 3rd February 2019, 04:37 PM

Great to hear you got it fixed!
I considered changing the big caps as well, but as they have quite an unusual shape and they still measured fine, I decided to leave them. Also as it is a preamp no large current peaks will be drawn from these reservoir caps so they are less critical as in a power amp.

ALRAINBOW · 4th February 2019, 11:03 AM

Guys need help for a cap to replace.
On the lower rear board near center there is a cap thst is brown plastic line it has a shell
It's next to a 100 uf cap. I did not replace this did not see what it's specs was.
Does anyone know. Help please

19th October 2018, 07:29 PM	#12
sgrossklass diyAudio Member Join Date: Nov 2004 Location: Germany	Good job. Quite impressive circuitry, the voltage regulators alone are pretty fancy. Output noise should be unspectacular but generally OK, there is only one stage of volume control by the looks of it. Looks like it should be easy enough to implement some bootstrapped cascode goodness via a capacitor across R150-D30 (and whatever the other channel equivalent is). Something in the double-digit µF range should do just fine, I think, maybe even a bit less if that results in turn-on issues. That should bring distortion down even further. I did spot one potential problem - when high gain mode is enabled, C55 is about an order of magnitude too small, so the amplifier would end up being on the bass-light side. (I presume it's a film cap?) Parallel some more capacitance if needed (maybe two 47-100µ back-to-back). With some more C in that spot, there'd be a realistic chance of bringing output noise down further as well. Right now it's by no means terrible but just slightly better than average for a dedicated pre, being hampered by moderately high feedback resistor values. Here are some values to bring the wideband noise floor down by about 3 dB: R140/141/142 = 1k0 / 154R / 4k22 and in order to avoid increasing output loading: R2 = R193 = 11-12k, matched / <1% (Should output loading be a noteworthy contributor to distortion, you can always move the input-side end of R193 to ground. The balanced output is still going to work, just 6 dB quieter and with some common-mode component. In the stock circuit, this would bring main amp output loading from about 3.7k to 14k. Not as much benefit when modified, of course.) Resulting noise floor should be little higher than 3 µV, good enough for 95-ish dB/W/m speakers with +30 dB worth of power amp following. That's 90 µV on the output at 44 dB of gain or good integrated amp level. Those with super high efficiency speakers - use an attenuator between pre and power amp or a variable input or generally lower gain (e.g. 22-26 dB) power amp, with the power amp not being too noisy in itself (good ones will manage around 30 µV). Top-end pres from the late '70s to early '90s would often be fitted with two-stage volume controls, usually employing 4-gang pots. Last edited by sgrossklass; 19th October 2018 at 07:38 PM.

3rd February 2019, 01:28 AM	#13
ALRAINBOW diyAudio Member Join Date: Dec 2013	Hello all. I have an issue with my no 26. It was fine till I moved it now one channel is lower. .it acts the same in se and bal both input and output. At one point if I moved the psu cable into the pre amp a bit it worked fine. Now it's always like this. I just finishe'd changing all the caps in the preamp . And ty for posting the schematic. I did test the voltages at the psu seems fine. Any ideas have any ideas ? Thanks in advance. Last edited by ALRAINBOW; 3rd February 2019 at 01:31 AM.

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12th October 2018, 11:05 AM	#11
Extreme_Boky diyAudio Member Join Date: Dec 2003	Very nice, thank you Prime3p14

3rd February 2019, 01:29 AM	#14
ALRAINBOW diyAudio Member Join Date: Dec 2013	Also once it's fixed any mods are very welcome. It's powering my no 33 amps. I need it working on balancertain mode .

3rd February 2019, 01:35 AM	#15
AVWERK diyAudio Member Join Date: Jan 2004 Location: So Calif.	A labor of love indeed. I would burn out way to early ! Regards David

3rd February 2019, 01:38 AM	#16
ALRAINBOW diyAudio Member Join Date: Dec 2013	Hahaha I just spent 6 hours taking it apart the pre and changed the caps. Found 3 bad caps too. I used a cap tester after I removed caps. But it still is the same lol.

3rd February 2019, 03:04 AM	#17
ALRAINBOW diyAudio Member Join Date: Dec 2013	Ok I have followed the psu from est psu to internal regulators all is ok so far. Oddly if use the pre for a headphone amp it's sounds fine Only when I connect it to my amps it's low on one channel I have done all the usual stuff reversed channels and cables I am now using a krell preamp all is ok. Any ideas ?

3rd February 2019, 05:24 AM	#18
ALRAINBOW diyAudio Member Join Date: Dec 2013	Guys a happy ending it was the Dip sws inside in doing the de cap I hit one somehow Set to max output all balanced out now. Also it has more low level details too. I'll print out the upgrade for the preamp and check it out. Thanks for the schematic very useful Wonder of I should change the large caps inside the psu case too. Any input. ??

3rd February 2019, 04:37 PM	#19
Prime3p14 diyAudio Member Join Date: Sep 2018	Great to hear you got it fixed! I considered changing the big caps as well, but as they have quite an unusual shape and they still measured fine, I decided to leave them. Also as it is a preamp no large current peaks will be drawn from these reservoir caps so they are less critical as in a power amp.

4th February 2019, 11:03 AM	#20
ALRAINBOW diyAudio Member Join Date: Dec 2013	Guys need help for a cap to replace. On the lower rear board near center there is a cap thst is brown plastic line it has a shell It's next to a 100 uf cap. I did not replace this did not see what it's specs was. Does anyone know. Help please