Idea for a dedicated Rhodes preamp, need help !

[PermO]

Actually... it was like that, but I took a grinder to it to shorten it.



I think it looks a bit silly, out of proportion.
Also the tiny box does not need a handle to carry it, the reason was to protect the pots and switches in case some bigfoot steps on it.

I think I like the clean look the most.

 

[L´Andratté]

I think I like the clean look the most.

Same with me, to protect the knobs it should have been on the other side of them?

PermO, you have infested me with DI-fever (or preamp). Thinking of buillding something coloured germanium-y, based on the topology below...hmm

 

[PermO]

Interesting ! Nice and simple circuit by the look of it...

1M input impedance and some germanium gain, no transformers... probably sounds awesome on bass and synths.

I once build a stereo germanium line amp running at an insert of my mixer "The Germanizer" , a bit of a nonsensible thing as it started with a 20dB pad followed by 20dB of germanium gain... it sounded very good, but it was a bad circuit, unstable and caused thermal runaway on the transistors over time and they blew... but the sound, it was really good.

So I sweep it with Easera, and it shows a flat line, it does not "sound" at all... but that's not what's happening in my perception ! So I play some music, sounds fine, I insert an extra circuit with all it's quircks and now the imaging improves, there's more depth, it's wider and the vocalist takes a step forward.
I cannot believe it is the added distortion that does this, this to me is highly intriguing, what is going on ?

So I might have a go at your circuit at some point.
Wise thing to keep it at +12v -12V as I googled my germanium transistors almost all of them were 30V max, the ASY77 were the only ones in my collection that could handle 60V.
I still need to look for an sillicium replacement to test as my ASY77 stock is limmited, prefer the same TO5 housing so it fits the socket but I have not got a clue, are all transistors "audio" or do some fare better than others ?
I know IRF510 will do the job, and sound good, but that's a form factor that will not fit the socket.

I know of a really nice collection of germanium transistors, (old broadcast lab stock) all tested and paired and stored... and I know they will trow all of these out soon as they are no longer allowed to repair anything older than 10 years.
The whole place is being reorganized at the moment, most techs got send home.
I have a friend working at that place, so we are keeping a close eye on that drawer.

 

[L´Andratté]

Sorry it´s really OT!
As is the circuit above is unity gain, need to modify the feedback path for a little AC-gain and why not try a transformer at the output?

But in the long run I really need to build one of these (I told you, it´s a fever):

 

[Pinokio]

No problem, I just ordered 20 very fancy boards from JLC PCB.
Probably here in two weeks. and hope I made no mistakes.
I can also provide you the vintage Beyerdynamic aux output transformer if you wish (it can easyly be bypassed on the circuit board)

Main out, very clean and punchy, aux out has a bit of a vintage flavour with the germanium transistor, baxandall EQ and the transformer.

Hi Permo,

Thanks great to hear, I can't drop you a PM on this forum.
I see you're based in Hilversum. Is there another way to contact you?

Thanks!
ps. most of the time im 30/40 minutes away from Hilversum

Gegroet.

 

[PermO]

I've send you a PM, I think you should be able to reply...

 

[PermO]

Yesterday I took some enclosures to a local powdercoater.
I had to stick to the classic RAL colors, so I went with RAL7034.
Also found a local service that can do UV printing.

Used the RAL RGB code for background and created an off white from that to finish my design.

I think this might be it;

 

[PermO]

I think this is it !



Best one so far I think...

 

[PermO]

 

[PermO]

So I did an attempt to get some numbers but I think my setup is insufficient to show the limits of this unit.

I connected the unit to my ADC input and used an old version of EaseRa to get some readings.

So about -140dB is what I get, but this is not the noise from the Instrument Amplifier, if I unplug the unit from the ADC I get the exact same readout.

So I decided to apply a bunch of gain from the Instrument Amplifier, it has 33dB of gain +6dB from the THAT1646 output driver, so here I am applying 39dB of gain going into the ADC (no input on the Instrument Amplifier) and I get this;


Aprox -120dB

With the applied 39dB of gain from the unit can I now assume it's noise level sits at -159dB ?

I got a poster sized piece of silver colored paper and did a beauty shot;

Super happy with this thing, sounds amazing.

 

[abbey road d enfer]

With the applied 39dB of gain from the unit can I now assume it's noise level sits at -159dB ?

-159 is the measurement in one frequency bin. Overall noise depends on the number of bins.
For 2 bins, it's +3 dB, for 4 bins it's +6 dB.For 64k bins, it's 48 dB more.

 

[TomKelly]

This looks like a great project. Are you planning on offering kits (full or partial)? Very interested in one if you do.

 

[PermO]

With all the labour and investment (this thing is not a cheap build) sourcing all the parts, drilling enclosures, powdercoating, UV printing, it will be a lot more attractive to sell it as a complete working unit.

Getting this thing boxed is quite a tedious effort as many things will have to allign perfectly, print, holes, circuitboard, jacks, rotary switches. It's all a supertight fit.

I do have some spare circuitboards and the fitting transformer for the aux output, the dip8 IC's are going out of style but they are still out there.
Only the jacks are PCB mounted so it will fit many enclosure styles, two people on this forum got a PCB from me to build their version of it.

 

[TomKelly]

Ah, completely appreciate that. I think I'd still be interested in a built unit! Would you be willing to sell me a PCB and the transformer?

Cheers

 

[PermO]

No problem, just send a PM and I'll hook you up.

Got schematic and parstlist so you should be good.