Mark Levinson 26 pre-amp schematics

Prime3p14

Member
2018-09-27 8:44 am
Hi fellow audio enthusiasts

I have drawn the schematics of my ML26 out of interest and it may be useful should I ever have to repair it :) I would like to share as it may be useful for others as well. Use at your own risk. It is correct to the best of my knowledge, some component type number were unreadable though, mostly diodes and a few transistors on heatsinks.

Greetings,

PrimeEvil
 

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PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
>> larger version?
> pictures got resized


Click the "4 arrows" icon in the corner of the image. See attached. It gets BIG.

They are quite large: first is 1792x2800. Bigger than your screen. I have to open in new tab or copy-off to an image view program.

The images are JPEG. For line-drawings and text, GIF will not fuzz-up the small details to reduce file size; clean black/white GIF typically comes out smaller than any legible JPEG.

PNG is another option but has several modes which can result in quite different filesizes.

PDF is yet another, and PDF viewers typically have good pan/zoom features. Additionally they can bind the several pages into one file. But PDF encoders are not universal.
 

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PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> I did upload as png, however somewhere in the process they got converted to jpg

Test of simple PNG attach --- it appears the forum software takes PNG verbatim.

I _do_ know what you mean. Another picture site takes my crisp GIF and JPEGs it.

Also browsers sometimes re-render an image at a different DPI.
 

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Last edited:
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.
 
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ALRAINBOW

Member
2013-12-08 3:03 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.
 
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ALRAINBOW

Member
2013-12-08 3:03 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

Member
2013-12-08 3:03 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

Member
2018-09-27 8:44 am
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.