Akai 707 Preamp Modification

[untune]

Half moved. we are in rented accommodation while our new place is finished.Yes, it should the point is to direct interference (hum) to the chassis instead of direct to the signal 0V.

The mumetal can itself is mounted directly to the chassis and therefore isn't isolated.

Cheers
Lee


That is fine.

Cheers

Ian

I've stayed renting for longer than I should have, purely because I don't want to deal with the stress of moving everything again. The more projects I get stuck into, the more junk I acquire which will unfortunately will have to travel with me  ;D

Thanks for the info - I'm going to tidy up the wiring tonight and shield things properly see if it cleans up the input. I've been testing with a dynamic (EV RE20) and the only other mic I have which is 150ohm and doesn't require phantom is a valve mic with it's own PSU - I'll test that and see how it goes.

Cheers!
Lee

 

[ruffrecords]

I suspect your hum problem stems in part from having too much gain. There's probably 20dB from the transformer and 30dB from each 6AU6 so there's a good chance you have around 80dB or more gain.

Cheers

Ian

 

[untune]

I suspect your hum problem stems in part from having too much gain. There's probably 20dB from the transformer and 30dB from each 6AU6 so there's a good chance you have around 80dB or more gain.

Cheers

Ian

I agree; I'm thinking I might isolate each input channel and have a switch between the EL84 and either the 1st 6AU6 (which is the mic transformer or a padded unbalanced line level input plus volume/tone control) or the 2nd 6AU6 (direct instrument level input.)

That should tame the gain to keep things reasonable and let me use different valves and a slightly different triode wiring scheme for each (1st is currently using the grounded plate and suppressor and screen as plate, 2nd is screen & suppressor tied to plate). I could also potentially use a different 7 pin preamp valve for one of them, but I think options are a bit limited really!

Cheers
Lee

 

[ruffrecords]

Sounds like a good plan.

Cheers

ian

 

[untune]

I had time to work on this last night; I altered it so that it can now be switched between V1 and V2, and it no longer forces V1 through V2. The transformer was properly wired with shielded cable for the signal. The good news is that it sounds much better, there is still some noise (this was to be expected anyway) but it's far, far more usable than it was before. I tested a few mics, and I think that it would be great on louder sources. With only one 6AU6 the gain was reduced and as a result I feel that there's not quite enough gain for my RE20 (which needs a lot of gain to begin with) on a speaking voice, but it would be more than enough for a bass or guitar cab, or a kick drum. Other, more sensitive mics seem good. It would have been great to use it on ribbons but I doubt there will be enough gain. Since my HT is different than I designed for, I should really go back and redeaw the load lines and perhaps adjust the cathode resistor.

The direct V2 channel works really well and has plenty of gain for a bass guitar, and the volume can be controlled from the instrument for a clean signal, or padded at the output for a dirtier more distorted signal. I'm thinking about adding a switchable -10dB pad for this input purely because there is a hole already drilled in the panel! Just a 1K log pot across the OT sec - would this benefit from a proper H-pad with resistors?

The only issue I still have is that synths have a horrendous buzzing noise when plugged into either the V1 line input or the V2 DI input. I had heard of this happening before but hadn't experienced it with other preamps. I imagine it's a side effect of the grounding scheme but I'm not sure whether it can be cured. A bass guitar doesn't have any buzz issues in either of these inputs.

Beyond that, I just need to figure out the VU meter situation and I can box it up!

Cheers
Lee

 

[ruffrecords]

I think boxing it up should help with the noise/hum levels. The high impedances in tube circuits make them prone to electrostatic field pick up which shielding cures.

Fundamental noise levels are dominated by the noise of the first stage so it is worth selecting this tube for low noise. The many 6AU6 tubes I tried a few years ago varied very widely in their noise characteristics but the best of them exhibited very low noise.

Don't worry too much about the operating point of the input stages as the signal level is small. The EL84 stage does however need to be biased properly to ensure its dissipation is within spec.

Cheers

ian

 

[smilan]

Hi Lee,
I've just finished to read this all the 6 pages of this thread and I've learned a lot from it.
So what are your conclusions?
Does this mic pre sounds good enough to you?
Do you recommends this project?
There's any other tips I should know before I starting this project?

 

[untune]

Hi Lee,
I've just finished to read this all the 6 pages of this thread and I've learned a lot from it.
So what are your conclusions?
Does this mic pre sounds good enough to you?
Do you recommends this project?
There's any other tips I should know before I starting this project?

Hi Smilan! I was a bit surprised to see a reply to this topic after so long... You know, the 707 pre is actually still sat behind me half assembled... I meant to make a few mods before I put it together but ran out of steam with electronic projects.

I felt that the first channel (mic and line) didn't have as much gain as I would have liked. The resistor arrangement for the line in was padding it down too much and I've wondered whether a line transformer would have been a good idea, since synths introduced sone buzz. The second channel, which was for a bass guitar directly into the valve sounded great to my ears.

I am yet to case it up with some proper shielding to get an idea of noise etc but it's still on my to do list. Are you attempting a similar project?

I also managed to track down a bit meter but again, didn't finish it. Maybe it's time to get it done!

 

[smilan]

Are you attempting a similar project?

Hi Lee,
Thanks for your reply,
I can get a here a 707 unit for quite cheap and I'm  wondering if it's worth the effort / time / extra money (for the transformers and other components).
Since I have few other pres, but nun of them is a valve pre I feels like this is the right time for me to get one (and if I can build one for my self and learn more about valves amplification it's even better.
For my needs the mic pre stage is the most relevant one, so if I can't modified this unit to a quality mux pre with about 60-70db of gain I think I'll pass this project.
Anyhow, let me know if you have different conclusions after finishing this project.