Akai 707 Mod Revisited

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untune

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https://groupdiy.com/index.php?topic=64477.0

Morning all!

The link above is the original thread that I started way back in December 2016... I felt like it was probably better to start a fresh one since the other was getting long and had multiple revisions.

To recap: I acquired an old Akai 707 tape machine and wanted to see if I could make a decent valve preamp from it, since there were a lot of popular mods for similar machines.  Over the course of a few months I ended up designing practically an entirely new circuit - my first foray into valve electronics with a LOT of help from the good folk here.

It needed boxing up and was put on the backburner with a few outstanding issues while I gathered bits.  I gradually drifted away from electronics projects and so I gathered pieces over the months - plywood for a new case, new grill cut for the faceplate, various knobs, valves, tolex to cover it in.  One particularly troublesome issue was that I couldn't find a VU meter to fit the unusual size hole cut in the face.  I remedied this by geting hold of a Sifam AL19, dismantling the original 'toy' VU meter - then modifying it so that the new meter would sit into the casing of the old one and slot perfectly back into place.

So a few nights ago, I decided to plug it in and see if everything still worked and it did indeed - and it sounded much better than I remember leaving it.  So I've decided while I have a bit of oomph to get it completed, I'll try to solve the last few niggling issues and get it closed off.

Basic layout is: Channel one is a mic input on XLR into Sowter 9610; separate line input on TS, padded with resistors.  Both into a switch into the first 6AU6 (triode). Then into an (apparently useless tone/high pass filter) then into a volume pot, then to a switch(2). 
Channel two is an instrument input on TS straight into another 6AU6 (triode), then straight into switch(2).
Switch(2) determines whether channel one or channel two goes into the next stage, which is a 6BQ5 (triode) into a Carnhill VTB 2291 on the output.

I've attached the last schematic I did which I believe was the way it was put together.

Here are the things I need to sort:

1) The line input.  Tried it with a synth last night and it actually sounded great but I recall another synth I tested it with a while back had a buzzing issue - I've pondered a line input transformer but space is tight.  The line input resistors were chosen because I believed I needed to pad it down a fair bit to prevent overloading the 6AU6.  If I remember right it's padding down to about 1/10 the input voltage.  I'm not sure if there's a better way to do this, and what kind of impedance matching problems I might have made.  Thinking about building an external 1:1 transformer box to isolate problematic line inputs like the aforementioned synth.

2) No phantom power on the mic input.  I can live without this, and may just build an external phantom supply - assuming there's not an easy way to get it in here.

3) Tone control does nothing.  I believed that C3 (0.1uF) and VR2 (50K) should technically form a variable high pass filter but it has never worked.  I've tried wiring it a few ways but each time it either does nothing, or acts like another volume pot.

4) VU meter.  Has 12v LEDs in to iluminate - can I utilise the 5.7v tap I have left on the PSU to supply this?  Guess I'll have to rectify the AC? Doesn't bother me if they're dim.  Also buffering the VU at the output - can I build a circuit to prevent distortion that would be caused by just putting it across the output?  Or would it be better to just put it across the output and stck a defeat switch in there for when I'm recording?

Thanks for reading :)
 

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fazer

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Nice I have 2 channels waiting to be modded so following your progress.  Also as I skimmed through your post I thought about the line in with tone control for rounding the buzz sound of a synth.  I think about this as a guitar player,    I find with a bright speaker/amp,  the tone control on a guitar single coil is extremely important to me.  On a mic input it maybe useless. 
 

untune

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fazer said:
Nice I have 2 channels waiting to be modded so following your progress.  Also as I skimmed through your post I thought about the line in with tone control for rounding the buzz sound of a synth.  I think about this as a guitar player,    I find with a bright speaker/amp,  the tone control on a guitar single coil is extremely important to me.  On a mic input it maybe useless.

Hi Fazer! Thanks for your reply.  I too have another Akai machine that I bought not long after this one (an M8 if I recall) that I was hoping to one day turn into a stereo valve compressor of some kind.

The tone control is a bit of an irritation now - I tried again to wire it differently last night but still no effect.  Starting to think that the 0.1uF might be too large a value for a hpf.  Pondering maybe a different type of filter, not sure.  My reasoning was that a high pass filter would be useful for taming low end rumble either from a mic or low synth notes, and also for counteracting proximity effect when micing bass cabs etc.

Interesting development though is that the synth that used to buzz now sounds fine so maybe I just had a bad ground when I last fired it up.  Might make the input transformer idea moot.

At the moment I'm just wondering how best to buffer the VU meter.  As for the lights, I think I can rectify the 5.7V heater tap to power the LEDs which, as far as I'm aware, are two surface mount LEDs inside the VU case with a 330ohm CL resistor.  So a diode and a cap should do it, I think?
 

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