Akai M8 Mic Preamp Conversion Build

[nedonnelly]

Hello all electron wizards!

I am planning on converting an Akai Reel to Reel Tape Machine M8 into 2 mic pres, and I wanted to start with some questions regarding features that I would like to have coming out of this build.

To start, here is the original M-7 Schematic:
View attachment M7-alt-LandR-cleaned-up-from-service-manual.png

Unfortunately I don't have any pictures of the actual unit as I am waiting on it to arrive, but I'm trying to get a jump on the project by asking some questions here.

Now on to the features that I would like to have.
1) +48v switch
2) Phase and pad switch
3) Balanced and unbalanced input
4) Balanced and unbalanced output

I've been watching a bunch of videos from Kyle Reid amplification to try to understand the possibilities of these R2R units. Here he is walking through the features:


From that video it seems the phase, pad switch, and unbalanced/balanced input are easily doable. However, it doesn't look like he has a balanced output anywhere on the preamp. I also don't see a phantom power anywhere.

So my questions are:

1. How manageable would it be to have both an XLR output and 1/4 in. output with this circuit?
2. Could phantom power be reasonably inserted into this circuit?

For what it's worth, it looks like TRC electronics makes a nice plug and play phantom power: https://trcelectronics.com/products/mean-well-rs-25-48.
Also, I see that someone at world pro audio chose to have a balanced output, but I'm unsure if there is also an unbalanced output. https://www.worldproaudio.com/WPA-Akai-M7-mod

Thanks for the help!

 

[C.B. - Boudio]

If you were to lift the speaker output from ground, you'd basically have a (very) low impedance balanced out (5-8 ohms or something) - but not really on a normal studio level (I don't know for sure how much 4W at 5 ohms is in dBu); some padding will be required I think. If there's a line out, that'll be then your 1/4 jack.
Also, I don't know how that lifting-from-ground will act with the NFB loop and/or the meter, which is after the output transformer.

Phantom power, it'll be blocked from the grid of the first valve by the capacitor (and if a balanced input were used, the input transformer), so it seems possible. Just make sure it's a very 'clean' supply, I suppose (the Meanwell is a SMPS, so I am not really sure how that will work with interference).

But there are many people here who are much, much more knowledged on it than I am!

 

[nedonnelly]

Just realized that the schematic has a boatload of extraneous information, and might be annoying for people to look at.

Here is what I have been looking at doing thanks to the information at lacinato.com:

 

[TimG]

Hello! I snagged this when I bought a Teracorder Deluxe I had hoped to convert to a preamp. It isn't a good model for conversion, as the tape head is the only input. Could be overcome...BUT!

I found this great guide on Akai/Roberts conversions. https://lacinato.com/cm/audio/robertsmod
Best models, options. - TimG

BACKGROUND

---

WARNING: This is very dangerous. Tube amps can easily kill you (250V-350V+ are exposed in these). Take it seriously, and don't try these modifications unless you are qualified. See also these safety notes in the build notes.

---

I did a normal "2-in-1" mod for both. I added a switch for the power tube section, used existing switches for the backlights in the meters, and for the 720A added an impedance selector switch for the mic input transformer (all very simple adjustments to the standard mod). I used Cinemag CMMI-10C input transformers on the 720A, and Jensen JT-115K-E's for the M7. I modified the original case to house the amps, which works great, but does take some time.

The 720A works fine for these mods: same tubes, same circuit, etc. There were some anomalies that I don't understand, but I suspect those can be attributed to my lack of experience, not the amps themselves.

Note: It is normal with these mods for each of the stages to be polarity reversed: meaning, an audio signal sent in will be "flipped" in polarity on the way out. Though basically harmless, this can be inconvenient if you're mixing the signal with other, non-flipped mics on the same source. You can flip the polarity of the XLR input (see details in the annotated instructions, linked just above), but the 1/4" jacks (including the 1/4" input on the EF86 stage, if you have one) will still be reversed.

Note also that the output impedance of at least the EF86 stage might not be what you expect... Going into a high-impedance input like some on audio interfaces (e.g. the 1/4" input of the Mic 1/2 jacks on a Presonus Firebox, which have 1M-Ohm input impedances for guitars, etc.) can result in loss of high frequency and high sensitivity to cable length (longer cables leading to more loss.) If you have any insight into this issue (e.g. what the output impedance of these units is, exactly), please contact me.

On the subject of loudness: used as a guitar amp, going into a 1x10 or 1x12 8-Ohm speaker cabinet (those are the sizes i've tried), these little 3-5 watt power amps are able to be respectably loud.



MODELS

Working models:

Roberts 770, Roberts 770A, Roberts 770X, Roberts 720 (very likely, see below), Roberts 720A, Akai M7, and Akai M8 are known to work. I did see one picture of a weird-looking 770, so I'm not sure those can always be guaranteed.

The Roberts 990 looks very likely to work (one person described it as "the same as M8").

The Akai M-7SP and Akai M-7SE look very likely to work, but that's unconfirmed. The SE apparently was the first with "crossfield" heads, and the SP might have just referred to external speakers that came with the unit, not sure. The Akai 707 seems very similar... not sure about it though.

The Roberts 997 is often conflated with the Akai M7. Looks the same.

Note: there is no reason I'm aware of to favor a particular model over another in terms of these mods -- they will all be functionally identical. There may be some slight differences in layout, slight differences in tube sub-type, etc, but there is no reason to choose one over the other besides the aesthetics of the dials, faceplate, etc. As far as I know, the differences between these models were all in the tape transport and other tape-related features that will be removed by the mods. I mention this because some people search only for an M8, thinking that it's "the best" when I'm not aware of any reason to do so. It's possible that there is some advantage to choosing the latest model: for example, perhaps the later date of manufacture means components are less likely to have failed. But the opposite could also be true if the manufacturing cut corners over time. Until someone reports in about such differences, I recommend you cast a wide net and find any unit you can if that is in good condition for a good price.

More info:

Roberts 770A, 770X and Akai M8 are all the same (says helpful internet person Adam: the only difference is the faceplate; the 770A came out in '63-64 and the 770X in '66-67). The 720A is the same as those but without some tape recorder features. The "770X SS" is a solid state version (no tubes, obviously not appropriate for these mods).

Roberts 770 and Akai M7 are the same. I think they are cheaper versions of the 770A/770X/M8 (but that should be irrelevant for the mods). The 720 is apparently some kind of cheaper version of those (akin to the 720A-to-770X/M8 relationship) but I'm not 100% sure that a 720 would work for these mods. I assume it would. Let me know if you know.

The Roberts 90C (90-C) might be an earlier, mono version... similar jacks and knobs, one had 12ad7 and ECC83; another was reported to have: "6AU6, 6AU6, 6X4, 6BQ5, 6AR5, 12AD7" (the inside pictures looked pretty different in some ways, pretty similar in others. Might be different enough to require some expertise to make it work. Not sure what the extra 6AU6 was for - it seemed to be mounted on some separate piece of the case?)

Models that will not work:

(It's been a while since I made this list, but I think these are all Roberts model numbers except where noted.)

The "definitely wrong, or you should assume it won't work" list includes: 5050X, 420XD, 333X, 450, 450A, 1720, 1721, 1725W-III, 80, 525, 530, 100, 95, 333X, 400X, 440, 5050XD, 420X, 650XD, 610X, 144, 1700, 1719, 1740X, 6000, 450, 1650, 1670, 770X SS, 771X, 778X, 800X, 808, Akai M4, Akai M5, Akai M9, Akai X-100D.

The "has or might have tubes but probably not workable with the standard mod" list includes: 1040 (has tubes), 1055 (2 X 12AT7, 2 X 6BM8, 6CA4), 1057 (tubes), 1600 (tubes), 1620 (tubes), 1630 (tubes, made in 1966 at least), 1640 (made in 1965 at least, has 6bm8, 12ax7, 12at7), 1725-8L III, 1725

Possibly-useful links with images of various models:

https://www.reelprosoundguys.com/roberts.html
https://reel2reeltexas.com/vinListA.html

 

[nedonnelly]

Hello! I snagged this when I bought a Teracorder Deluxe I had hoped to convert to a preamp. It isn't a good model for conversion, as the tape head is the only input. Could be overcome...BUT!

I found this great guide on Akai/Roberts conversions. https://lacinato.com/cm/audio/robertsmod
Best models, options. - TimG

BACKGROUND

---

WARNING: This is very dangerous. Tube amps can easily kill you (250V-350V+ are exposed in these). Take it seriously, and don't try these modifications unless you are qualified. See also these safety notes in the build notes.

---

I did a normal "2-in-1" mod for both. I added a switch for the power tube section, used existing switches for the backlights in the meters, and for the 720A added an impedance selector switch for the mic input transformer (all very simple adjustments to the standard mod). I used Cinemag CMMI-10C input transformers on the 720A, and Jensen JT-115K-E's for the M7. I modified the original case to house the amps, which works great, but does take some time.

The 720A works fine for these mods: same tubes, same circuit, etc. There were some anomalies that I don't understand, but I suspect those can be attributed to my lack of experience, not the amps themselves.

Note: It is normal with these mods for each of the stages to be polarity reversed: meaning, an audio signal sent in will be "flipped" in polarity on the way out. Though basically harmless, this can be inconvenient if you're mixing the signal with other, non-flipped mics on the same source. You can flip the polarity of the XLR input (see details in the annotated instructions, linked just above), but the 1/4" jacks (including the 1/4" input on the EF86 stage, if you have one) will still be reversed.

Note also that the output impedance of at least the EF86 stage might not be what you expect... Going into a high-impedance input like some on audio interfaces (e.g. the 1/4" input of the Mic 1/2 jacks on a Presonus Firebox, which have 1M-Ohm input impedances for guitars, etc.) can result in loss of high frequency and high sensitivity to cable length (longer cables leading to more loss.) If you have any insight into this issue (e.g. what the output impedance of these units is, exactly), please contact me.

On the subject of loudness: used as a guitar amp, going into a 1x10 or 1x12 8-Ohm speaker cabinet (those are the sizes i've tried), these little 3-5 watt power amps are able to be respectably loud.



MODELS

Working models:

Roberts 770, Roberts 770A, Roberts 770X, Roberts 720 (very likely, see below), Roberts 720A, Akai M7, and Akai M8 are known to work. I did see one picture of a weird-looking 770, so I'm not sure those can always be guaranteed.

The Roberts 990 looks very likely to work (one person described it as "the same as M8").

The Akai M-7SP and Akai M-7SE look very likely to work, but that's unconfirmed. The SE apparently was the first with "crossfield" heads, and the SP might have just referred to external speakers that came with the unit, not sure. The Akai 707 seems very similar... not sure about it though.

The Roberts 997 is often conflated with the Akai M7. Looks the same.

Note: there is no reason I'm aware of to favor a particular model over another in terms of these mods -- they will all be functionally identical. There may be some slight differences in layout, slight differences in tube sub-type, etc, but there is no reason to choose one over the other besides the aesthetics of the dials, faceplate, etc. As far as I know, the differences between these models were all in the tape transport and other tape-related features that will be removed by the mods. I mention this because some people search only for an M8, thinking that it's "the best" when I'm not aware of any reason to do so. It's possible that there is some advantage to choosing the latest model: for example, perhaps the later date of manufacture means components are less likely to have failed. But the opposite could also be true if the manufacturing cut corners over time. Until someone reports in about such differences, I recommend you cast a wide net and find any unit you can if that is in good condition for a good price.

More info:

Roberts 770A, 770X and Akai M8 are all the same (says helpful internet person Adam: the only difference is the faceplate; the 770A came out in '63-64 and the 770X in '66-67). The 720A is the same as those but without some tape recorder features. The "770X SS" is a solid state version (no tubes, obviously not appropriate for these mods).

Roberts 770 and Akai M7 are the same. I think they are cheaper versions of the 770A/770X/M8 (but that should be irrelevant for the mods). The 720 is apparently some kind of cheaper version of those (akin to the 720A-to-770X/M8 relationship) but I'm not 100% sure that a 720 would work for these mods. I assume it would. Let me know if you know.

The Roberts 90C (90-C) might be an earlier, mono version... similar jacks and knobs, one had 12ad7 and ECC83; another was reported to have: "6AU6, 6AU6, 6X4, 6BQ5, 6AR5, 12AD7" (the inside pictures looked pretty different in some ways, pretty similar in others. Might be different enough to require some expertise to make it work. Not sure what the extra 6AU6 was for - it seemed to be mounted on some separate piece of the case?)

Models that will not work:

(It's been a while since I made this list, but I think these are all Roberts model numbers except where noted.)

The "definitely wrong, or you should assume it won't work" list includes: 5050X, 420XD, 333X, 450, 450A, 1720, 1721, 1725W-III, 80, 525, 530, 100, 95, 333X, 400X, 440, 5050XD, 420X, 650XD, 610X, 144, 1700, 1719, 1740X, 6000, 450, 1650, 1670, 770X SS, 771X, 778X, 800X, 808, Akai M4, Akai M5, Akai M9, Akai X-100D.

The "has or might have tubes but probably not workable with the standard mod" list includes: 1040 (has tubes), 1055 (2 X 12AT7, 2 X 6BM8, 6CA4), 1057 (tubes), 1600 (tubes), 1620 (tubes), 1630 (tubes, made in 1966 at least), 1640 (made in 1965 at least, has 6bm8, 12ax7, 12at7), 1725-8L III, 1725

Possibly-useful links with images of various models:

https://www.reelprosoundguys.com/roberts.html
https://reel2reeltexas.com/vinListA.html

Have read this guide more times than I can count

The Wayback links at the bottom that link to the original beyondsanity productions links are super helpful. When I start the conversion those will probably be my guide.

 

[OneRoomStudio]

I don't know for sure how much 4W at 5 ohms is in dBu

4W into 5 ohms is 0.894Vrms (P=V^2/R).

0.894Vrms = +1.245dBu

 

[Brian Roth]

4W into 5 ohms is 0.894Vrms (P=V^2/R).

0.894Vrms = +1.245dBu

Random question in this thread.....is that voltage level at clipping for this unit? If so, not suitable for pro audio.

Bri

 

[moamps]

4W into 5 ohms is 0.894Vrms (P=V^2/R).

0.894Vrms = +1.245dBu

Hmmm...

 

[OneRoomStudio]

Hmmm...

Haha, thank you for that prod. I must have been overly tired when I did that calculation (I have a 14-month old baby).

4W into 5 ohms is 4.472Vrms. (4x5=20 and sqrt20=4.472)

4.472Vrms is +15.23dBu

Plenty!