Akai M-8 R2R SE pentode amplifiers
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- Start date
SteveBOmaha
New Member
roberts 770 as phono preamps?
Hey guys,
Thanks for the replies. Sorry I went missing... the roberts pres sounds great. the ef86 has nice high end detail without being harsh. They compress when driven. The 12ax7 seem to have a little upper midrange lift but still sound sweet. I have tried a couple unknown input transformers and noticed this affects the frequency response. I have used API preamps alongside these amps. The Roberts aren't as fast with transient response it seems, but in a good way, almost like a mild limiter. They rule as a bass DI. hi
Currently as I am recording less, I put the 6bq5 tube back and are using these as my main integrated stereo amp through the 12ax7 input. I constructed a U pad to tame the signal from my computer interface and the amps sound fantastic. I believe by modifying the ef86 stage into a separate pre, the eq network required for accurate phono playback was removed. I would like figure out how to mod this stage into a switchable phono stage...
I am very much a beginner DIYer...this project is my first. So if some of these problems and solutions may seem elementary to some.
Hey guys,
Thanks for the replies. Sorry I went missing... the roberts pres sounds great. the ef86 has nice high end detail without being harsh. They compress when driven. The 12ax7 seem to have a little upper midrange lift but still sound sweet. I have tried a couple unknown input transformers and noticed this affects the frequency response. I have used API preamps alongside these amps. The Roberts aren't as fast with transient response it seems, but in a good way, almost like a mild limiter. They rule as a bass DI. hi
Currently as I am recording less, I put the 6bq5 tube back and are using these as my main integrated stereo amp through the 12ax7 input. I constructed a U pad to tame the signal from my computer interface and the amps sound fantastic. I believe by modifying the ef86 stage into a separate pre, the eq network required for accurate phono playback was removed. I would like figure out how to mod this stage into a switchable phono stage...
I am very much a beginner DIYer...this project is my first. So if some of these problems and solutions may seem elementary to some.
sloober
Super Member
It is a very over complicated circuit but it works pretty well. I would just use it as is, or gut it and come up with a simple front and rear circuit. Mine as a guitar amp is pretty cool, but I am considering gutting it and building some sort of hybrid combination of Fender schematics retaining the transformer and 6BQ5.
Bummer is, that when using the mic or other inputs, it is in RECORD mode meaning the tone control is non-functional. With a scratch build you could retain the retro looks but build the functions you want, like input control, tone control and phono stage. Another nice feature is the components are mostly on a separate board that can be re-built.
Bummer is, that when using the mic or other inputs, it is in RECORD mode meaning the tone control is non-functional. With a scratch build you could retain the retro looks but build the functions you want, like input control, tone control and phono stage. Another nice feature is the components are mostly on a separate board that can be re-built.
Hi,
this thread is very old, nevertheless I try to "revive" it.
I've got several questions about the AKAI M8 pres:
1. I dont know which tubes to take out since all I can read is a shade of the brand (NEC).
2. I will use the units as mic pres in my lowend studio.
Are the "pre" out the right outputs to use? Or should I prefer the
line outs inside the pres which were originally wired to the DIN output?
3. I didnt have a closer look to my M7 pres; can I do the same modifications to
them, provided by tracker-x?
By the way: Many thanx to you tracker-x for sharing your knowledge!
Thanx!
Philipp
this thread is very old, nevertheless I try to "revive" it.
I've got several questions about the AKAI M8 pres:
1. I dont know which tubes to take out since all I can read is a shade of the brand (NEC).
2. I will use the units as mic pres in my lowend studio.
Are the "pre" out the right outputs to use? Or should I prefer the
line outs inside the pres which were originally wired to the DIN output?
3. I didnt have a closer look to my M7 pres; can I do the same modifications to
them, provided by tracker-x?
By the way: Many thanx to you tracker-x for sharing your knowledge!
Thanx!
Philipp
On the bottom of my rack you can see space for the XY to 110V Transformers.
Since I am located in Europe, this question:
The blue wire seems to be the 250V on M8 and 230V on M7.
I connected blue, green gray and black.
Black seems to be ground, blue 250/230V and grey like zero.
Is that correct?
Or is there a way to leave away the 250/230V converter transformers?
Ph
Since I am located in Europe, this question:
The blue wire seems to be the 250V on M8 and 230V on M7.
I connected blue, green gray and black.
Black seems to be ground, blue 250/230V and grey like zero.
Is that correct?
Or is there a way to leave away the 250/230V converter transformers?
Ph
janikphoto
Lunatic Member
So, I followed the six pin connector into the unit, and my blue seems to be ground on the power switch, the red seems to be 117 volt positive on the transformer. Now for my question... I'm left with white, yellow, green, and black. Are these possibly inputs from the r2r that could be tapped for an rca from the rear? Would it still have the same fidelity/response? It would be nice to leave 1/4" input alone up front. If I can't tap from these wires, my plan is to remove the 1/4" and use its wires to do rca in back. Same plan to remove the speaker output and put a plug in back. Then, I'll probably put a rounded head bolt in the empty holes up front, making them look like fastening points. Thoughts?
janikphoto
Lunatic Member
doug s.
Super Member
here's a q&a i had w/donn tarris, who supposedly knows quite a lot about these amps:
q:
> hi,
>
> i saw your website for the roberts tape deck amps, &
> was a bit interested in what you do w/these amps. i
> have a mint pair of akai m8 amps that i am
> presently using in an audio system. i have figured
> out how to power them up from the m8's original power
> transformer. i take inputs from my preamp into the
> inputs at the top left of each amp, & i have wired a
> 1/4" fone plug to normal speaker binding posts for the
> speaker outputs. they sound really nice into
> 97db-efficient 14 ohm speakers. the gain on them is a
> *ton*, i have to barely open the wolume pots to get a
> lot of gain. the tone is fully max’d out to get flat
> frequency response for a normal stereo rig.
>
> what i am wondering, is what, if anything, can be done
> to mod these for better performance as normal stereo
> amps. when i got them, they looked like they had
> never been used – really amazing. i am not interested
> in using them as preamps, tho i may be interested in
> retaining their volume pots...
>
> thanks,
>
> doug s.
a:
Hi Doug,
By default, the top switch on the units is spring loaded to the
playback position, which is accessed on the top left input. This goes
to the EF86 input tube, which is set up for a "ton" of gain, to
amplify the signal from the tape deck head. The eq is also set for
playback based on their output response curve, which means you need
to have the tone control full on.
It would work better to remove the top bar and spring assembly to
allow the switch on the top to be set to the record position, which
would then have you plugging into the input second from the bottom on
the left, the line input. You could also go into the lower left
input, normally for a high impedance microphone, giving you more
gain. Your frequency response should be better through this circuit.
The speaker output on these is normally 8 ohms, so your amp is
working a bit hard to deliver to 14 ohm speakers, but should be ok.
Let me know if this helps.
Cheers,
Donn
and followup:
> thanks, this makes sense. i was wondering why the
> line input dint work - i have to set the spring loaded
> switch to record, eh? i noticed this switch disables
> the amps when i manually move it, & the feed is going
> into the top left input. :>)
>
> so, disabling the "playback" switch & setting it to
> "record", & then using one of the other two inputs
> will give me better sound? how will it affect the
> tone control? it *does* sound flat w/the tone max'd
> out as i am now using it.
tone isn't in the record circuit, it plays no part, just flat... In
theory, you shouldn't need the tone control in the playback.
> i surmised the amp was 8 ohms, & i wasn't getting the
> power out of it i should be, w/14 ohm speakers - do
> the output transformers have a 16 ohm tap? do you
> know what the power rating of the amps are?
I think the amps are about 8 watts, not sure if there's a tap for 16
ohm, I don't believe so.
> also, what (if any) mods/upgrades would you recommend
> for using as power amps - i know your site seems
> dedicated to using these as preamps...
I don't think you'd need much more. You could replace the power caps,
etc, clean all the contacts, pots...
doug s.
q:
> hi,
>
> i saw your website for the roberts tape deck amps, &
> was a bit interested in what you do w/these amps. i
> have a mint pair of akai m8 amps that i am
> presently using in an audio system. i have figured
> out how to power them up from the m8's original power
> transformer. i take inputs from my preamp into the
> inputs at the top left of each amp, & i have wired a
> 1/4" fone plug to normal speaker binding posts for the
> speaker outputs. they sound really nice into
> 97db-efficient 14 ohm speakers. the gain on them is a
> *ton*, i have to barely open the wolume pots to get a
> lot of gain. the tone is fully max’d out to get flat
> frequency response for a normal stereo rig.
>
> what i am wondering, is what, if anything, can be done
> to mod these for better performance as normal stereo
> amps. when i got them, they looked like they had
> never been used – really amazing. i am not interested
> in using them as preamps, tho i may be interested in
> retaining their volume pots...
>
> thanks,
>
> doug s.
a:
Hi Doug,
By default, the top switch on the units is spring loaded to the
playback position, which is accessed on the top left input. This goes
to the EF86 input tube, which is set up for a "ton" of gain, to
amplify the signal from the tape deck head. The eq is also set for
playback based on their output response curve, which means you need
to have the tone control full on.
It would work better to remove the top bar and spring assembly to
allow the switch on the top to be set to the record position, which
would then have you plugging into the input second from the bottom on
the left, the line input. You could also go into the lower left
input, normally for a high impedance microphone, giving you more
gain. Your frequency response should be better through this circuit.
The speaker output on these is normally 8 ohms, so your amp is
working a bit hard to deliver to 14 ohm speakers, but should be ok.
Let me know if this helps.
Cheers,
Donn
and followup:
> thanks, this makes sense. i was wondering why the
> line input dint work - i have to set the spring loaded
> switch to record, eh? i noticed this switch disables
> the amps when i manually move it, & the feed is going
> into the top left input. :>)
>
> so, disabling the "playback" switch & setting it to
> "record", & then using one of the other two inputs
> will give me better sound? how will it affect the
> tone control? it *does* sound flat w/the tone max'd
> out as i am now using it.
tone isn't in the record circuit, it plays no part, just flat... In
theory, you shouldn't need the tone control in the playback.
> i surmised the amp was 8 ohms, & i wasn't getting the
> power out of it i should be, w/14 ohm speakers - do
> the output transformers have a 16 ohm tap? do you
> know what the power rating of the amps are?
I think the amps are about 8 watts, not sure if there's a tap for 16
ohm, I don't believe so.
> also, what (if any) mods/upgrades would you recommend
> for using as power amps - i know your site seems
> dedicated to using these as preamps...
I don't think you'd need much more. You could replace the power caps,
etc, clean all the contacts, pots...
doug s.
aux.tech.pdx
New Member
Thanks Doug, glad you posted this.
janikphoto
Lunatic Member
So, the main reason not to use the input from the reel is the tone circuit and gain aren't suited for most other audio devices? While working on mine last night, I moved the unit out of record and lugged an input into the back (where the reel used to send its signal to) and the wires for the old internal speakers were used. It sounded ok, but the tone switch needed to be full-on to sound right.
I eventually clipped the RCA connector in back, de-soldered the wires for the internal speaker, and am using the front line in and speaker out. Does anyone know if there's a good place to wire up the record indicator bulb to be used as a "power on" indicator? I'm guessing it'll be a 6.3v bulb and not a 110v.
I eventually clipped the RCA connector in back, de-soldered the wires for the internal speaker, and am using the front line in and speaker out. Does anyone know if there's a good place to wire up the record indicator bulb to be used as a "power on" indicator? I'm guessing it'll be a 6.3v bulb and not a 110v.
janikphoto
Lunatic Member
I've still got my father's m-8 in working condition and I enjoyed it as a r2r. I think $100 is a fair price for a working unit. These aren't throw-away pieces like they used to be. You'll need to pay a few bucks to grab one! The last one I bought had issues with one channel and I paid $40.
This looks like it might be what you want.
http://lacinato.com/cm/audio/robertsmod
If not, I have save the documents and can email them to you if you like.
John
http://lacinato.com/cm/audio/robertsmod
If not, I have save the documents and can email them to you if you like.
John
hi im new to this mod, and i got everything done except i dont know where to connect the bulb to indicate power on instead of record on. can you tell me exactly where to conect the red wire with the resister and black?If you want to leave the record bulb where it is, just connect it to the 6.3 volt heater supply to the tubes. 6.3 volt bulb of course but your meter will tell you this!
Please don't guess!
thanks
Conrad was a friend of mine. I actually have this amp that he built.
Ok, I have some bad news, and some good news:
The link to Conrad's Tube Stereo page is dead: http://rummageads.com/tstereo/ There is something about a server crash on the home page. It looks like they lost everything.
But here is the good news: I found a version to be still around on the wayback machine.
http://web.archive.org/web/20050331085218/http://rummageads.com/tstereo/
This was a really cool site. This guy would have been right at home on AK, but if I recall correctly, he died from cancer several years ago.
