Request for suggestions. Akai M8 mono amp schematic validation. - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 12th June 2012, 08:49 AM   #1
xpl1c1t is offline xpl1c1t  United States
diyAudio Member
 
Join Date: Jan 2008
Post Request for suggestions. Akai M8 mono amp schematic validation.

Aloha,

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.

Click the image to open in full size.

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!

Last edited by xpl1c1t; 12th June 2012 at 09:06 AM.
  Reply With Quote
Old 12th June 2012, 10:27 AM   #2
xpl1c1t is offline xpl1c1t  United States
diyAudio Member
 
Join Date: Jan 2008
For reference, here is a diagram of the original circuit;

Click the image to open in full size.
  Reply With Quote
Old 12th June 2012, 04:31 PM   #3
diyAudio Member
 
Join Date: Jan 2008
Location: Southern California
Hi,

I started a similar thread two weeks ago,

Need help in modifying Roberts 770 monoblocks

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.
  Reply With Quote
Old 12th June 2012, 06:20 PM   #4
xpl1c1t is offline xpl1c1t  United States
diyAudio Member
 
Join Date: Jan 2008
avincenty,

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)

:smile:
  Reply With Quote
Old 12th June 2012, 09:34 PM   #5
diyAudio Member
 
Join Date: Jan 2008
Location: Southern California
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.
  Reply With Quote
Old 14th June 2012, 03:18 PM   #6
xpl1c1t is offline xpl1c1t  United States
diyAudio Member
 
Join Date: Jan 2008
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?
  Reply With Quote
Old 19th June 2012, 04:58 PM   #7
xpl1c1t is offline xpl1c1t  United States
diyAudio Member
 
Join Date: Jan 2008
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.
Attached Images
File Type: jpg circuit.jpg (111.7 KB, 218 views)
  Reply With Quote
Old 29th June 2012, 04:52 PM   #8
xpl1c1t is offline xpl1c1t  United States
diyAudio Member
 
Join Date: Jan 2008
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.
Attached Images
File Type: jpg schematic.jpg (94.7 KB, 192 views)

Last edited by xpl1c1t; 29th June 2012 at 04:56 PM.
  Reply With Quote
Old 29th June 2012, 05:16 PM   #9
frugal-phile(tm)
diyAudio Moderator
 
planet10's Avatar
 
Join Date: Oct 2001
Location: Victoria, BC, NA, Sol III
Blog Entries: 5
With that circuit you will likely have way too much gain. A single AX7 stage should be more than sufficient.

dave
__________________
community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
  Reply With Quote
Old 29th June 2012, 05:32 PM   #10
xpl1c1t is offline xpl1c1t  United States
diyAudio Member
 
Join Date: Jan 2008
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Akai M8 rebuild/restore/will-this-work? pjanda1 Tubes / Valves 23 2nd July 2014 03:17 AM
Help power up akai M8 amps Eric123 Tubes / Valves 6 18th April 2012 11:29 AM
Akai M8 rebuild up and running! pjanda1 Tubes / Valves 5 8th May 2010 02:28 PM
Akai M8 Modding Service Required Jaye Tubes / Valves 1 5th January 2010 06:09 AM
AKAI M8 Tube R-R input problem abmoggy Tubes / Valves 21 3rd July 2009 11:42 PM


New To Site? Need Help?

All times are GMT. The time now is 01:47 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2