Idea for a dedicated Rhodes preamp, need help !

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Brian Roth

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Could you please explaing what the "the Tascam problem" is?
It's not only a Tascam problem, but also Avid and numerous other "product problems". It consists of an output stage design with two opamps (typically) operating 180 degrees "out of phase" with each other to try and create a balanced output line driver. Problem arises when a system unbalances that line by driving one of the "legs" into ground/0V. That poor opamp will do all it can to drive whatever current it can into that short circuit and often induces a distorted mess into the 0V system.

In addition, the output voltage will drop by 6 dB because one leg is missing it's voltage contribution.

Bri
 

PermO

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Ah ok, so the circuit only screws things up when it's not properly terminated.
I have always learned to short a differential input's leg if it's not connected, but never short a differential output, because you don't know what is there in most cases.

So.. I've got some little bags of sand from Reichelt and the fancy box has the THAT differential output driver.
DSCF3133.JPG

And I am putting together a little project box for this;
DSCF3130.JPG
I think it's pretty schwifty...

When the first lockdowns were announced I figured I would spend a lot of time at home so I did a bulk buy of alieexpress stuff to keep me busy, anything that looked kinda decent, switches, knops, VU's, jacks, boxes, relais. Now I am really happy I did that as I have plenty stuff to build many things. Though not all is perfect, this VU comes from a set that had one broken, so I figured I just use it here, cool thing is, the buffered driver has two inputs so I put a little toggle switch next to the VU so I can read two different points in the circuit.
Input / output jack on the front, output XLR on the back, 6 pos 2 pole Lorlin can be used for attenuation duties, small 4 pole switch can be used to eveluate different cap or resistor values while playing.
The switch on the right takes the spare 12V output from the VU driver board and could be used to activate a couple of relais.
It's easy to swap these 16x10 cm cards as they just slide into the grooves on the side, I love these boxes.
Stuff won't fall apart when experimenting, I can pick it up and bring it to a friends studio, and with the lid on everything is properly shielded.
All the AC stuff and powerswitch are in the back, so at one point I could drop a piece of steel between the PSU and the audiocircuit for extra shielding if needed.

I'm not committing to a PSU yet, I'll do some more reading, download all the datasheets for the transistors I've got and take it from there.

I think it was an afternoon well spend so far.

Edit;

Ok, to avoid the Tascam problem I really want to use the THAT chip for balanced output, and it needs a differential psu (max +18 -18) so I will need that anyway, OPA134 will be smoking though... the FET's I have will be fine on 15V...
If I create an extra possitive tap with a LM317 and a trimmer I will have every possible voltage I will need from 5 to 15V, it will make experimenting very easy.
 
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Whoops

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It's not only a Tascam problem, but also Avid and numerous other "product problems". It consists of an output stage design with two opamps (typically) operating 180 degrees "out of phase" with each other to try and create a balanced output line driver. Problem arises when a system unbalances that line by driving one of the "legs" into ground/0V. That poor opamp will do all it can to drive whatever current it can into that short circuit and often induces a distorted mess into the 0V system.

In addition, the output voltage will drop by 6 dB because one leg is missing it's voltage contribution.

Bri

Thank you so much for explaining Brian
 

PermO

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Ok, did some more reading and found that I read the wrong information... probably had a datasheet for a small SMD OPA134, as the troughhole version will run perfectly fine on +15 / -15 V.
So, I won't need an additional PSU stage for experimenting as all my sillicon can handle the voltage.

This leaves me some room in the back near the output jack for something funky :poop:

Finnished the PSU last night;
DSCF3136.JPG

And made a cover to keep things save, but I had to do a cutout for the AC receptable, and that did leave the only lethal part (the fuse bridge on the AC receptable) exposed for a big fat finger to touch... that made no sense, so I fixed it.
It connects to ground when pushed into place by a nudge with some solid wire that connects to the XLR chassis bolt.

DSCF3137.JPG

I think my experimentationbox is comming together quite nicely...

I have got a nice little workshop full of stuff and nice tools, I can't help it, first my brain starts rushing, then my hands start to do things and there's no stopping... it just happens 😀
 

PermO

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Almost finnished my protobox, powersupply is working +15.05 and -15.02, the 12V output from the driverboard can handle the relays so I put the funky :poop: 1:1 isolation transformer between bypass relays, seemed like a cool option to play with.
It comes from a radio broadcast system, used as line isolator, I guess it's 600 Ohms (the paintjob is mine, these came unbranded) meassures 550 at the primary and about 700 at the secondary, they are designed to mount in small 8 pin tube sockets. there's 5 little colorcoded leads visible at it's mounting pins, black 0 for ground, brown 1 for prim 1 red 2 for prim 2 orange 3 for sec 1 and yellow 4 for sec 2.
They sound really nice when pushed a bit, I only have 3 of these and a whole stack of Lundahls, but I prefer these mystery transformers.
DSCF3153.JPG

Almost there, almost time to play and learn...
 

PermO

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Anybody got a clue on who could have made these ?
Probably European, my guess was Lundahl but I could not find anything there with these socket pins.

DSCF3138.JPG
 

abbey road d enfer

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It comes from a radio broadcast system, used as line isolator, I guess it's 600 Ohms (the paintjob is mine, these came unbranded) meassures 550 at the primary and about 700 at the secondary,
According to these DC measurements, I would think the nominal impedance is 10k.
 

PermO

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Thanks ! ... I did a misleading paintjob 😀

Am I correct in my thinking that a 10K load is easyer to drive for most circuits compared to a 600 Ohm load ?
(Let's asume the "driver" is a Behringer with a Tascam problem and a Avid complex :confused: or something similar)
 

PermO

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Ok, I have decided to start with the chips, checking those datasheets it seems to be a realy easy way to do this.
So I will be running OPA134 into THAT1646, the datasheet suggests no coupling cap, just go directly in, cool.
So some powersupply local small caps at the inputs and some nonpolar caps for common mode offset reduction.
That's easy, exept for 1 thing I'm clueless about, pin 1 and 8 on OPA134 "Offset trim" ...where do these go ?



DSCF3159.JPG
I don't have any nonpolar electrolitics but I guess a pair of large film caps will do the trick also.

Anybody has a clue on pin 1 and 8 ?


(edit)
Found some more info and figured they don't need to go anywhere... cool.
Bed time, build it tomorrow...
 
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PermO

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Well, no smokin' FET's but the THAT chip also appears to be dead and the inputs of the VU driver board appear to be blown out...

So.

This is done.


(edit !)

So a couple hours later and some more rigourous testing... the Protobox is still ok, everythig is functioning, both VU channels still work. Now I will have to find out if OPA killed THAT... I will set up the output stage, put a probe with a tonegen at the input and see if anything will still come out at both ends.
 
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weiss

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Following this.

Especially i'm interested in the comparison to the Alembic preamp. I also run my Rhodes through it.
 

PermO

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Ok, and the good news is, the THAT balanced output driver is still fine, I probed pin 4 with 0.7V from a tonegen and it outputs on both pin 1 and 8... so, it's actually working.

No damage done exept OPA is dead.... carry on.

Oh, and that book, it is comming my way.... I think I really need that 😀
 

JohnRoberts

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I was going to suggest that the THAT IC might not be dead. They invest a lot of silicon into protection circuitry on every I/O pin.
===
For today's too much information, several years ago I talked with THAT engineers (actually only one THAT engineer) about reconfiguring one of their popular interface chips with all the precision resistors onboard into a dedicated synthesized current source. The precision resistors would have guaranteed a current source with high output impedance/compliance. They had all the precision resistors my current source topology needed, and a semi-custom metallization layer to re-configure them differently was not huge bucks (maybe a couple $10k). BUT the stopper for them was that my current source configuration resulted in one additional I/O pin and their substrate protection wells were all used up. They wouldn't make/sell an IC with even one unprotected I/O pin...

JR
 

PermO

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Yeah, succes ! I have "a" preamp.

I replaced OPA134 with the FET input stage / follower mentioned before;
DSCF3161.JPG
This time I decided to put small bypass caps on all the powerrail connections.
Following the THAT datasheet I removed the coupling cap but I connect the two stages via my stepped attenuator wich is wired like this;
DSCF3163.JPG
The 10M input resistor sits on a small 4 pole switch wit a 3.3M, 1M and a 470K.

It works and it is nice and quiet, no hum or hiss, but it's output is very very low, with the gain switch on max hitting the piano hard I barely hit -20dB on my mixer ch VU (with its gain in the notched zero position) I can apply a ton of gain on the mixer, and it was still nice and quiet, but that's not the output level I'm after, I want this thing to be able to hit a transformer balanced input on a compressor.. hard !
(as a line amp this setup would probably function just fine, but not on a Rhodes piano)

And I got my transformer wired backwards :confused: ..I followed the colorcoding but should have been wiser and look at the DC readings, I'm hitting the 550 Ohm winding instead of the 700 Ohm winding which takes another 10dB or so, don't know, it drops off the scale of my meterbridge.

So.

GAIN !

I want gain... my guess.. R2 and R3... need to get smaller, I put 3.3K and 2.7K in as this got mentioned earlyer.. though I don't know, should both values be the same or should this "imbalance" be there ?

Drop them to 1K and see what happens ?
 

JohnRoberts

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That is configured as a follower (unity gain). If you want voltage gain you need a NF circuit. Add a resistor between the collector of the PNP transistor and source of JFET.

A 2.7k there will give you a gain of 2x.

JR
 

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