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Old 10th January 2010, 03:33 AM   #1
pjanda1 is offline pjanda1  United States
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Default Akai M8 rebuild/restore/will-this-work?

I just picked up an Akai M8 R2R. I replaced the 'lytics and coupling caps, chopped the box down, turned it upside down and tossed them back in it. It is a darn neat little amp (monoblocks with choke filtered supplies!), and the chassis got me to thinkin'.

I am still pondering a bi-amped open baffle speaker setup. This little guy has preamp outputs. How happy would it be driving both it's output section and my PP 6V6 amp (50K input impedance) with some simple passive filters? It has much more gain than I need, so insertion loss isn't a problem. I was thinking of biamping with SS on the bottom, but SE on top and PP on bottom sounds like much more fun.

The second problem is that it doesn't sound as good as my El Cheapo (Eli Duttman's PP 6V6 triode amp). I'm glad, because the El Cheapo is huge (mine is choke input and motor runs) and took plenty of time and $$. But, if I'm going to use this thing on fullrangers, I need it to sound better than it does (it sounds fine, but not good enough). Would some parts upgrades and a conversion to triode cut it, or do I need to consider building in a new circuit? I wouldn't be opposed to doing something like an RH84, but I'd need to put a preamp in front of it. The chassis allows some flexibility in that regard. But, if the rebuild gets too complicated, I may as well sell these guys and build the 12B4 preamp I as thinking about (though then I'd still have SS on the bottom. . .)

So A: Will I be happy biamping with the preouts

and

B: Do I tweak the stock circuit, or start anew?

I couldn't find an online schematic to post, but I figured a few folks might be familiar with them. It's a fairly typical EF86 (not using) 12AX7, SEP EL 84 R2R amp. I'll run to a copy shop and scan the schematic if folks want to see it.

Paul
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Old 10th January 2010, 10:05 AM   #2
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Dude,

Before trying something "sophisticated", swap the OEM PSU diodes out and snubbed UF4007s in. A simple major reduction in PSU "hash" level could go a long way towards making the sound better.
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Old 10th January 2010, 02:05 PM   #3
pjanda1 is offline pjanda1  United States
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They are tube rectified: 6X4's. I know I need to get a schematic up. What do you think about the preamp out/passive filter idea?

Paul
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Old 10th January 2010, 04:23 PM   #4
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Quote:
They are tube rectified: 6X4's.
So much for the better "sand" diode idea.

Quote:
What do you think about the preamp out/passive filter idea?
I find the possibilities of impedance mismatches galore disturbing. Buffering the passive blocks, as Marchand does in his tubed crossovers, seems better to me.

Quote:
I need it to sound better than it does (it sounds fine, but not good enough). Would some parts upgrades and a conversion to triode cut it, or do I need to consider building in a new circuit?
You could change things along the lines of the RH84. FWIW, I'd add some GNFB to linearize the O/P trafo. Switching to LED bias for the voltage amplifier goes hand in glove with that concept. LR8 regulating g2 B+ is another possible tweak to the RH84 concept.
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Old 11th January 2010, 10:36 PM   #5
pjanda1 is offline pjanda1  United States
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Since an M8 schematic is nowhere to be found on the web, I figured I'd toss one up here. I cropped it into three parts so I could post it at the full resolution of the scan. Let me know what you think of the circuit, and especially the preamp ouputs capabilities.

Paul
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File Type: jpg M8_schematic_ch1.jpg (871.1 KB, 737 views)
File Type: jpg M8_schematic_ch2.jpg (919.5 KB, 663 views)
File Type: jpg M8_schematic_connections.jpg (680.6 KB, 622 views)
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Old 11th January 2010, 10:40 PM   #6
pjanda1 is offline pjanda1  United States
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Quote:
Originally Posted by Eli Duttman View Post
S
I find the possibilities of impedance mismatches galore disturbing. Buffering the passive blocks, as Marchand does in his tubed crossovers, seems better to me.
Eli, I've already got more tubes than I need in the amps. What about building a buffer into them that could drive anything? Maybe volume control>buffer circuit>preamp outputs>RH84 etc?
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Old 12th January 2010, 12:35 AM   #7
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Originally Posted by pjanda1 View Post
Eli, I've already got more tubes than I need in the amps. What about building a buffer into them that could drive anything? Maybe volume control>buffer circuit>preamp outputs>RH84 etc?

PJ,

That makes sense to me. The buffered volume control will be master. Set and forget volume adjustments on each amp are needed for "seamless" integration.

I can't say I've gone over the schematic with a fine toothed comb, but I didn't notice NFB of some kind around the EL84.

The 6AR5 is a small power pentode that draws 400 mA. of heater current. Right there, you have the heater current for 1 section of an ECC99 wired as a cathode follower, which is a kick butt buffer. Set alternate sections up in each channel, to allow for flip/flop when service life ends. You'll go YEARS between outright replacement. In the R2R, the 6AR5 is serving as the bias/erase oscillator. Recycle the EF86 sockets for the ECC99s.
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Old 12th January 2010, 02:35 AM   #8
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Originally Posted by pjanda1 View Post
The second problem is that it doesn't sound as good as my El Cheapo (Eli Duttman's PP 6V6 triode amp).
My main amplifier for nearly the last two years has been a pair of these that I stripped down and rebuilt on a new chassis. The design is very simple; an EF86 driving a SE 6V6 (pentode) in each channel. I used all the original iron. To keep the octal theme (6V6), I used brown-base 6X5s rather than the original 6X4s. Each channel is lined up side-by-side on the chassis with the EF86s in the front, followed by the 6V6s and 6X5s.

The sound quality of these amps is extremely good; you just have to get all the tape recorder junk out of the way. You also have to give that 12AD7 (12AX7) the boot. The original design used the 12AD7 to drive the output, but the EF86 absolutely decimates it in this application. Where the triode is dull and lifeless, the EF86 carries nuances and transparency that can almost overwhelm. As it has turned out, my speakers aren't quite up to the task. I have spent many late nights listening to these through Grados, astounded by the exquisite tonality of these inexpensive amps. The only other amps I've experienced this with are my 211 SETs.
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Old 12th January 2010, 03:17 AM   #9
pjanda1 is offline pjanda1  United States
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Now that is pretty! That snug fit is right up my alley. I'm trying to avoid a new chassis though. My wife and I both actually like the look of the M8's as is! I'd love to see your schematic. I'd consider 6AQ5's as the sockets would work out.

Heater current for the pre isn't a problem at all. In addition to the 6AR5, I assume I'll ultimately end up loosing another triode. I don't think I need a pre and two halves of a dual triode to drive an EL84.

With my wimpier sources, the EC has just enough gain. I'm afraid a cathode follower and the possibility of a passive filter might be be problematic. How about a 12B4 pre like I was asking about last month? I couldn't fit a fancy power supply, and I don't have as much B+ available as some folks use. I do intend to lose the 12AX7 as soon as possible, but it may be towards the end of this project. It is a speaker project afterall . . . FWIW, I'm exploring the possibility of using some sort of Behringer DSP to do the woofer crossover. They tend to have input impedances from 10-25K unbalanced, so the pre would need to drive that along with the RH84 or whatever I end up doing for the top amp.

Paul
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Old 12th January 2010, 03:39 AM   #10
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I have a total of 6 of these (1 pair in a tape deck i want to restore).

EF86 > EL84 was what i was going to do.

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