Latest AKAI/Roberts mods "The Boris"

[RodC]

I have been working on another version of my AKAI/Roberts pre mods. I?m naming this one ?The Boris?. An engineer in London contacted me and wanted a pair of Roberts pres modified, with a few twists. After several emails here is what we ended up with. It has 2 channels and is still powered by a tube rectifier.

Preamp 1:
- EF86 Tube
- Mic input wit Cinemag CMMI-7 C input transformer http://cinemag.biz/mic_input/CMMI-7C.pdf
- This transformer has a 50ohm tap for those low impedance Ribbon mics!
- Switch to select 200ohm or 50ohm mic input (Ribbon mic option)
- High impedance input (100K)
- Gain control
- Output level control (So you can push the pre if you want)


Preamp 2
- 12AX7 Tube (or most varities of the 12xx7 family currently has NEC 12AD7)
- Mic input with Shure M68 input transformer
- High impedance input (500K)
- Gain control
- Output level control.

You can gang the 2 togeter, as with my other versions, if you want lots O gain. (Just jumper the output of one stage to the high impedance input of the other stage)

This version also maintains the 6BQ5 (EL84) output if you decide you want a low power tube amp. (Speaker level)

Here is a quick MP3 I stuck together. The LEFT channel is Preamp 1 (EF86), the RIGHT channel is Preamp 2 (12AD7). (So you can really mute each side to hear the difference) The acustic was recorded with 2 GXL1200 condensors placed as close together as possible and pointed between the sound hole and the 12 fret.

The electric guitar and snare were recorded with the same setup, The LEFT channel is Preamp 1 (EF86), the RIGHT channel is Preamp 2 (12AD7). HOWEVER we used 2 SM57s. The mics were just off center of the cone for the gutiar and about 4? away for the snare. This is a pretty ringy snare.

There are no effects on the tracks, of course the acustic needs some compression.

The 12ax7 stage is a bit sensitive for the stuff I record, so it breaks up a bit too much for me on the loud stuff. I think that Boris will like it as it is. For my stuff I would prob tame it down a bit, but that can always be done with a lower gain tube. I like to use a 12DW7 in these situations, they are unbalanced, and the first stage has very little gain.

*** >> Here is the MP3: <<***
AkaiRobertsTheBoris.mp3

[Mudcloth]

You're killing me with these mod's. This is some really cool stuff you're doing. A friend of mine wants to mod his but hasn't found the m67 trannies yet. I should tell him to go with the cinemag's. Nicely done.
-Matt

[RodC]

The cinemags are def an improvement. They have other models, but the CMMI-7C is awsome if you have ribbons.

The cinemag is much smoother on the top end and you can really see/hear the bass response of it in sustain type sounds.

If you use this tranny you have to change the Gain pot to 100K to better match the tranny.

[ThunderFaceWizardHead]

The new mods sound great, any chance you'll be posting a schematic? Also, I didn't see a price on the Cinemag website, wondering if you had it?

[RodC]

When I get it all done I'll try to post the changes to go from my "2 in one" to this. They are pretty close, just some mods to get the cinemag in there and some output controls.

I think they cost around $40 each, not sure if he had to buy several to get that price or not?

[RodC]

BTW ThunderFaceWizardHead, I just noticed your post asking about the cinemags on the other thread, sorry I just noticed that. I have been tied up a lot lately and not been on the board much.

If you get to a point in your mods that you are ready for these trannys and I havnt posted an updated schematic, drop me an email and I will scan you a sketch and help you out.

Email me anytime if I dont respond to a post.

[Antero]

I just got a pair of these preamps for pretty cheap and I'm going to try my hand at your mods, so I too would totally love to see the schems for these when you get the chance.

[RodC]

Here is a draft schematic, I'm still tweaking a bit. You may want to run the input trannys through a switch on the 1/4" jacks to disconnect them when the high impedance inputs are used. Also, the output level controls are only effective in about 1/4" of their turn, I expected this and was willling to put up with it to get the rather constant 500K impedance on the output.



AkaiM7RightSchematic _MOD_BorisModel.jpg

[abechap024]

Hi! I've completed these mods Thanks to the detailed instrucitons, and thank you! Me being on a very limited budget, These AKia are the best preamps I've gotten for <$200! Sound SWEET! and using them as a guitar amp makes a deffinitly Awesome sounding recording amp. I was Impressed - hooked up to an 8ohm 2x12 the Akia kicked some sound out! Anyway down to my question. I love the preamps I used the shure input trannys and they sound great - but they do have some hum - not a lot but a little, and I think it might be because I didn't use shielded wire when rewiring the short connections inbetween tubes? but I was wondering if I wanted to insert an Output transformer, to help reduce hum and balance the outputs - what kind of tranny would you reccomend? a 600/600 ?? I'm not tooo savvy on how trannys totally work yet, but i'm eager to learn. Thanks for any help!~
AC

[RodC]

Glad you got them done!

Some tips, def use shielded cable for the inputs. I isolate the jacks or only ground one end of the cable to prevent ground loops.

You also may need to change the filter caps, mine are dead quiet even with the old caps. Also, if you primarly use them for preamps, pull the power tubes, this will ligten the load on the powersupply a great deal, making it easier for the diode and the filter cap to do it job.

Check all the grounds very well, the routing of all the grounds are different on every model. Make sure they are all connected by a good connection and not the frame.

Remember these are unbalanced outputs, use unbalanced cables or the - of the equipment you are going into willl be floating.

Does it hum when you only hook up the outputs? Does both the EF86 stage and the 12ax7 hum?

You could use a 600 to 600 for the output, if you maintain the cathode follower. If you decide to use an output tranny you may want to investigate some other output design. The cathode follower is designed to work without a tranny.

[abechap024]

As a reply to the last post - the mic pres themselves are actually very quiet. I realized it was my guitar that was the noisy element in the mix! Anyway I appreciate your advice on the trannys, I think those can wait. But my mind keeps on moving to other questions, I apologize. I recently realized what an awesome free program rtanalyzer is as I went through all my preamps, and got noise and specs for them, and I noticed on both AKAI the wavs distort un evenly, the bottem of the waveform distorts quit a bit more before the top of the wavform! The Mic pres sound great, But I am curious on how rebiasing the pres might lead to overall better preformance. I've looked everywhere I can think of for info on how to bias a tube preamp, and the only thing I learned was how much I have to learn about electronics!!! Is there a resistor I could replace with a trim pot to adjust the bias? Is bias even the right word for uneven wav clipping? Any thoughts would be most helpful! I am in your debt!
AC

[RodC]

As a reply to the last post - the mic pres themselves are actually very quiet. I realized it was my guitar that was the noisy element in the mix! Anyway I appreciate your advice on the trannys, I think those can wait. But my mind keeps on moving to other questions, I apologize. I recently realized what an awesome free program rtanalyzer is as I went through all my preamps, and got noise and specs for them, and I noticed on both AKAI the wavs distort un evenly, the bottem of the waveform distorts quit a bit more before the top of the wavform! The Mic pres sound great, But I am curious on how rebiasing the pres might lead to overall better preformance. I've looked everywhere I can think of for info on how to bias a tube preamp, and the only thing I learned was how much I have to learn about electronics!!! Is there a resistor I could replace with a trim pot to adjust the bias? Is bias even the right word for uneven wav clipping? Any thoughts would be most helpful! I am in your debt!
AC

The uneven wave clipping is sorta the nature of single ended cathode follower amps. Its sorta the sound of single ended distortion in many cases. If you replace the cathode bypass caps with larger values you may get a bit more performance out of it. The distortion you are seeing is sometimes hard to detect with the ear.

Do some googling for single ended tube. Cathode bypass and asymmetrical clipping. That may help you out a bit.

[RodC]

Im about to ship 2 more of these to England. I have received several emails from others building their own with success.

I wanted to post an update here, I try to check my directions everytime I start one of these. I found a mistake on the Slide Switch removal section. If you look at step 6 it refers to AK5, this should be AK3. Its just used for location of a connection so its not too serious. I will be updating the doc and FTP ing it up soon.

[AK Audio]

Anyone know what a decent current price is on these preamps? I have no idea what they go for these days.