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xpl1c1t 12th June 2012 08:49 AM

Request for suggestions. Akai M8 mono amp schematic validation.

My electrical engineering experience stretches way back to last week Sunday when I happened to find a Akai M8 out on these beautiful amplifier-rich streets of San Francisco (HK, Marantz, now Akai found). Turns out, with the help of these forums and some elbow grease i have arrived at the following circuit physically, I have done all of the circuitry reduction noticeable in this schematic and am asking for advice for changes and assurance i haven't done something whacked.

The amp sounds good at volume but there is a audible buzz @ 0-gain using the only headphones i have (Fostex orthodynamics, these are high-impedance drivers), and I intend to recap the amps with the following;

1x Orange Drop 715 Series .001F / 600V
1x Orange Drop 715 Series .010F / 600V
1x Orange Drop 715 Series .022F / 600V
2x Sprague Atom 25F / 25V
3x Sprague Atom 20F / 350V

Very much appreciated! :geezer:

xpl1c1t 12th June 2012 10:27 AM

For reference, here is a diagram of the original circuit;

avincenty 12th June 2012 04:31 PM


I started a similar thread two weeks ago,

In my case the open loop response is not flat and requires Negative feedback from the secondary. In the case of pentode connected it sounds much better with the feedback connection. I tested different values for the pentode feedback and a resistor between 50k and 60k worked best.

Definitely replace the caps, the non oil caps were bad and affected the bias of the second half of the 12ad7.

I did not try with headphones on mine, but with speakers it is silent. I only tested with the built in speakers and vintage alnico 4 inch drivers i had on the bench at the time.

xpl1c1t 12th June 2012 06:20 PM


Just making sure, you are suggesting I place a 50-60k resistor between the following points?

Pt1 (between R14 and 12ad7 secondary cathode)
Pt2 (between Transformer 2 and the signal lead for the voltage meter)


avincenty 12th June 2012 09:34 PM

Yes, it basically replaces AK4 (without the connection to switch).

If you have access to a signal generator and a rms voltmeter (or oscilloscope) you can verify the frequency response.

xpl1c1t 14th June 2012 03:18 PM

I have been recommended to to increase the pi filter capacitors to 40uF, the cathode coupling capacitors to .1uF and the cathode bypass capacitors to 100uF. These should decrease ripple, and increase bass responce, respectively.

Im also very interested to know if i will need some sort of input coupling capacitor, or will it be ok with the resistor-only input currently on here?

xpl1c1t 19th June 2012 04:58 PM

1 Attachment(s)
Well, I'm doing my best to get acquainted with tube amps but try as i might I have not found the right resource to solve this puzzle. I have been able to model the circuit in LTSpice and am getting some very disturbing output. First off, the power supply output (on this circuit only, with others it tests great) the output appears as a sawtooth wave with a delta of 5-6 volts. There is something garbaging up the works here that i cannot see.

The attached image is the Spice model i'm working with.

Any assistance would be appreciated.

xpl1c1t 29th June 2012 04:52 PM

1 Attachment(s)
Well I've spent some time reading the forums and various sources of wisdom and have arrived at this final circuit which I expect to perform very well. Again, if anyone has any suggestions for alteration I'd love to hear your advice.

The power caps will be GENTEQ GEM III motor runs.
Cathode bypass caps will be Kemet T262 solid tantalums.
Coupling caps will be Russian teflons.
Resistors will be RCD wirewounds.
I plan to invest in two Jensen 4:1 step-down input transformers so I may use a variety of sources. (My DAC outputs 2.1VRMS)
I also plan to invest in a EMI filtering power plug, a Delta 06EK3.

Thats all for now.

planet10 29th June 2012 05:16 PM

With that circuit you will likely have way too much gain. A single AX7 stage should be more than sufficient.


xpl1c1t 29th June 2012 05:32 PM

I'm thinking; eventually i'll plug in a guitar or something with an extremely small signal, so for direct input from a DAC i'll use a transformer.

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