Idea for a dedicated Rhodes preamp, need help !

[PermO]

Ok, more crazy PermO Rhodes ramblings...

This is a Rhodes MKI;

So evey hump is a little coil actually and they are all in series and there's 73 of them.
The lid on these things is plastic, I could not put my Moog which takes 230AC on top as it caused interference.

So I did this;

Yup, that works... Moog sits on top, no problem.. So the ring is tied to ground at the preamp input.
I prefer the sound directly from the bridge anyway, instead of those two dodgy pots with a cap.

So now that I have this, and am "designing" this super special preamp.... why not use it ?
This is not about keeping it low cost or simple, I want the best noise perfromance and gain I can get.

Do microphones always tie one end of the capsule to ground ?

Does it make sense to creat a a differential input stage for ultra low noise performance ?
Would an input transformer be ideal in this case ?

Rant mode off... thanks for partaking !

 

[PermO]

Hmmm my previous thoughts on the output transformer are incorrect, it's not reversed wired.
If a secondary has more windings to compensate for transformerloss the side with the highest DC reading (700R) is the secondary.
And the colorcoding is correct, makes sense.
Glad it did not swap it...

But, howcome do I now have a 10dB loss on this transformer ?
Is it, because my levels are so low the transformer becomes less efficient ?

 

[abbey road d enfer]

Do microphones always tie one end of the capsule to ground ?

Actually none. Mics are usually kept floating. Only teh body is connected to the cable shield.

Does it make sense to creat a a differential input stage for ultra low noise performance ?

It is debatable. If the preamp is close to the pickup, no. Didderential (balanced) connection cancels interference that is sensed by both wires; if they're kept short there is not much interference.
Differential is usually noisier than an equivalent unbalanced stage, by 3dB.

Would an input transformer be ideal in this case ?

The problem is that teh pickup is relatively high impedance. You would need a transformer with a nominal impedance of about 50 kohm.

 

[abbey road d enfer]

But, howcome do I now have a 10dB loss on this transformer ?
Is it, because my levels are so low the transformer becomes less efficient ?

Mismatch is the answer.
The pickup has a rather high inductance. I believe it's about 5-10H. When it meets a transformer with 2H primary inductance, you loose 10-15dB from the start.
Voltage divider equations apply to inductors (and capacitors) as well.

 

[PermO]

I this case it is not on the pickups, it is a 10k:10k line isolator, driven from the THAT balanced output, it takes away about 10dB and sucks all the low end out.
This does not happen when I drive them from a mixer output, these sound very nice actually.

But now it craps out for some reason.

So something's off with my output stage, I hought the THAT driver would handle a tansformer just perfect.
It is wired just as the example in the datasheet, schematic on previous page.

As the source of the Rhodes is nothing but a large coil it sort of felt logical for me to keep it floating and avoid groundconnections.
But indeed long lines need to be avoided at that end, and a fully balanced circuit (which would be nice to have) would probably need all it's components matched, that's a lot of extra work and trouble to go trough if it makes no sense.

So now I like to figure out what is going on with my balanced line driver, why does it crap out on the transformer ?
It sounds fine without the transformer.

 

[abbey road d enfer]

I this case it is not on the pickups, it is a 10k:10k line isolator, driven from the THAT balanced output, it takes away about 10dB and sucks all the low end out.

What is it connected to? Mic, line or instrument input?
How is the pickup connected?
Ther are too manuy possibilities of mismatch here. Draw a schematic.

 

[PermO]

I'm just focussing on the balanced output stage now, as I will have to come up with a different input stage that actually provides me some gain... like 30dB or so.

I guess my output stage would be fine like this;

(The transformer is on a bypass switch, it is in the proto-box for circuit test purposes to see if a circuit can drive a transformer well)

I figured this would be fine, but it results in a huge level drop and all low end gone ?

The output sounds fine when the transformer is not engaged.

 

[PermO]

The problem is that teh pickup is relatively high impedance. You would need a transformer with a nominal impedance of about 50 kohm.

That's probably the reason passive DI's are a mismatch .... the RNDI goes really well with the Rhodes.

 

[abbey road d enfer]

I'm just focussing on the balanced output stage now, as I will have to come up with a different input stage that actually provides me some gain... like 30dB or so.

I guess my output stage would be fine like this;
View attachment 82447
(The transformer is on a bypass switch, it is in the proto-box for circuit test purposes to see if a circuit can drive a transformer well)

I figured this would be fine, but it results in a huge level drop and all low end gone ?

The output sounds fine when the transformer is not engaged.

What do you connect it to?
If it's a mic input, it's a large mismatch.

 

[PermO]

Hmmm,... he's right again

Now that I have removed the input stage completely from the board.
Probe pin 4 (input) with a tonegen @100Hz set to 0.7V output.
And connect a small active PC speaker (for test duties) to the output XLR it sounds fine, no level drop when engaging the transformer and 100Hz comes out fine.

I did not expect the input stage could cause such behaviour at the output side as it did sound fine without the transformer playing my bass,.. (connected to a transformerless balanced input) ...engaged the transformer and the whole thing collapsed.

Ok, so my output stage is fine, I'm planning to stick with the 1646 for this build.

Now my focus is to find a nice low noise high impedance input stage, that provides a ton of gain and has a low impedance outptut to connect to the 1646...

 

[PermO]

What do you connect it to?
If it's a mic input, it's a large mismatch.

During the listeningtest it was connected to a balanced lin input of my mixer.
It sounded fine (but very low (no) gain) so I crancked the gain on the mixer input.
Played my bass (passive pickups) with the FET BJT circuit as input stage, got a decent sound, engaged the output transformer between the 1646 and my mixer channel and the whole thing collapsed, huge level drop, all low end gone.

 

[PermO]

Took the unit back to my studio setup, connected the XLR output with a 50ft cable to my patchpanel (mixer ch input) and put a tonegen at pin 4 of the 1646...
With fader at zero and gain at zero I set the tonegen to read 0dB on my ch VU and calibrated the VU driverboard in the proto-box to read 0VU also, playing with the output level of the tonegen I see the cheap Chinese VU isn't too bad actually, it shows the same reading as my LED VU bar on the mixer.

To estimate a bandwith of this stage I played with the tonegen and watched the ch VU on my mixer, well the 1646 probably has a more extended range than my tonegen as it still read 0dB at 100KHz... the mystery transformer gives up at around 35KHz and starts to drop below 30Hz ... not too bad I guess.

So, my output stage is ok.

 

[Whoops]

That's probably the reason passive DI's are a mismatch .... the RNDI goes really well with the Rhodes.

The input impedance of the RNDI is 2Mega

 

[abbey road d enfer]

2 Megohm is not a necessity. A Rhodes plugged direct into a guitar amp with 500k impedance sounds great.
In particular, I don't think the 10 Megohm used in the DynoMyPiano has any serious justification. It can't do no harm, but I think it's there because the mktg dept could brag about it.

 

[ccaudle]

with the FET BJT circuit as input stage, got a decent sound, engaged the output transformer between the 1646 and my mixer channel and the whole thing collapsed,

The whole point of That 1646 is to replace a transformer. Why are you using a fancy cross-coupled output stage and then putting some unknown transformer after it? With an output transformer you might as well just use a dual op-amp to drive it.

Now my focus is to find a nice low noise high impedance input stage, that provides a ton of gain

Your OPA134 would do that if you connected it right and didn't blow up the input stage. I'm surprised there was so little comment on the op-amp based schematic you posted, it was obviously not going to work even if you wired it up as drawn. Add two more resistors and it would have been fine.

What do you consider "line level" for output? Does it need to hit +20 dBu or is lower OK? That will define the amount of gain you need. My quick scratchpad estimation is you would need about 18dB gain to get to 0 dBu, so maybe 35dB or 40dB total gain. The 1646 has 6 dB, so that drops you to 30 dB or 35 dB in the front end. 30 dB closed loop on the OPA134 would still give you about 30 dB of loop gain at 10kHz, so should be pretty usable.
Note that estimation is based on a post I found that asked someone to check the output level from a Rhodes, so he connected an oscilloscope and hit a big 8 note chord and got peak levels of about 300mV. I have seen other sources that say the first transient can be much closer to 1V, so you may have to experiment to see how much gain is actually needed.
Note that if you distort an OPA134 front end (applies to any op-amp) the sound will get nasty quickly, there is no soft overload like with a tube circuit or some JFET circuits. If you mentioned early I did not catch whether you just wanted clean gain, or you wanted something to add a little distortion along the way like you might get from a tube or open-loop JFET front end.

 

[rock soderstrom]

Well, as I said before. There's more than one way to skin a cat. (H-E-L-P!)

One of my favorites is this circuit as a starting point. You can adapt this very well to your own needs.
For example, I made the source capacitors and the input impedance switchable. This is a very flexible and colorful MicPre and DI solution and its quite easy to build. The only drawback is that you have to preselect the JFETs so your driver stage is not getting too hot. But this can also be customized...




Here my test rig.

 

[PermO]

The whole point of That 1646 is to replace a transformer. Why are you using a fancy cross-coupled output stage and then putting some unknown transformer after it? With an output transformer you might as well just use a dual op-amp to drive it.

The proto-box is not the preamp, it's a box for experimenting, with easy connectivity and I put a transformer with bypassrelays in there so I can present it as a load when experimenting with circuits.
So when I figured my circuit, the proto-box will remain the proto-box, the 1646 will be my output driver.

Your OPA134 would do that if you connected it right and didn't blow up the input stage. I'm surprised there was so little comment on the op-amp based schematic you posted, it was obviously not going to work even if you wired it up as drawn. Add two more resistors and it would have been fine.

Yes, I know... now, someone mentioned the closed loop, but that was after the fact.
There was no mentioning of this in the datasheet, the 1646 datasheet did provide all the information needed to make it work. I have no experience with sillicon, I have only build point to point tried and tested tube designs.

What do you consider "line level" for output? Does it need to hit +20 dBu or is lower OK? That will define the amount of gain you need. My quick scratchpad estimation is you would need about 18dB gain to get to 0 dBu, so maybe 35dB or 40dB total gain. The 1646 has 6 dB, so that drops you to 30 dB or 35 dB in the front end. 30 dB closed loop on the OPA134 would still give you about 30 dB of loop gain at 10kHz, so should be pretty usable.

Well, OPA134 is dead, but yes 30dB / 35dB of gain should do the trick.

Note that estimation is based on a post I found that asked someone to check the output level from a Rhodes, so he connected an oscilloscope and hit a big 8 note chord and got peak levels of about 300mV. I have seen other sources that say the first transient can be much closer to 1V, so you may have to experiment to see how much gain is actually needed.

I'm not a loud 8note chord hitting player.
There's many ways to set op a Rhodes piano, and there's variations, mine is a old MKI with the wooden mechanics inside, it's quite a heavy monster to play and it's set up more towards the "bell" sound instead of the midrange "barking" sound.

Note that if you distort an OPA134 front end (applies to any op-amp) the sound will get nasty quickly, there is no soft overload like with a tube circuit or some JFET circuits. If you mentioned early I did not catch whether you just wanted clean gain, or you wanted something to add a little distortion along the way like you might get from a tube or open-loop JFET front end.

I don't know, this is an experiment, theory and schematics don't tell me anything about how it will sound in my setup, with my piano, so I will have to try several options and test if I like them.
I'm not looking for heavy color or distortion, but it does not have to be perfectly flat if it sounds right.
I'll drop another OPA134 in my basket with my next purchase.
But I'm thinking something with a FET will cause the next smoke.

 

[5v333]

I appretiate the fact that you are experimenting with circuits that are new to you. Also the sketching with pen and paper. You hit an obstacle and you prob learnd something from it. Look up inverting and non inverting opamp stages.

 

[ccaudle]

The proto-box is not the preamp

My misunderstanding then, probably I was skimming the older posts too quickly. I think I understand what you want now.

There was no mentioning of this in the datasheet

Op-amps are mostly standardized, the datasheets will not show all the basic usage information, same as transistor datasheets.
But figure 28 from the datasheet is basically what you want. It does not show the resistor from input to ground (or mid-supply reference if you are not using bipolar supplies), be sure to keep that from your original schematic. There are other things you would want to consider for a robust commercial style design, possibly extra input protection, RFI ingress protection, things like that, but figure 28 is the basic non-inverting amplifier configuration.

Figure 32 is the layout recommendation for that configuration. The schematic in figure 32 which labels the resistors Rf and Rg is the most common way to refer to those resistors if you are comparing with other texts (the feedback resistor from ground to inverting input, and the grounded resistor from inverting input to ground or reference voltage).

Well, OPA134 is dead, but yes 30dB / 35dB of gain should do the trick.

Are dB conversions familiar for you, or do you need assistance determining the gain ratio and appropriate resistors?

Are you sure the OPA134 is dead? Were you being literal in your earlier post that it "lit up a cigar," it literally glowed red?

I'm not looking for heavy color or distortion, but it does not have to be perfectly flat if it sounds right.

The OPA134 family is known for low distortion and clean sound, should be good for a clean preamp.
Also available as a dual in case you want to buffer the output for your pedals. Probably you will have to attenuate the signal if you are using typical guitar pedals, they would expect lower level. Something like the output of the first amp into the input of the second amp as a unity gain follower, which drives a resistor voltage divider to give the output level and impedance you want.

 

[PermO]

My misunderstanding then, probably I was skimming the older posts too quickly. I think I understand what you want now.

It started as a brainfart, and when putting the box togheter I figured this would be a nice thing to have for experimentation as it keeps things solid and has a good groundingscheme.

Op-amps are mostly standardized, the datasheets will not show all the basic usage information, same as transistor datasheets.
But figure 28 from the datasheet is basically what you want. It does not show the resistor from input to ground (or mid-supply reference if you are not using bipolar supplies), be sure to keep that from your original schematic. There are other things you would want to consider for a robust commercial style design, possibly extra input protection, RFI ingress protection, things like that, but figure 28 is the basic non-inverting amplifier configuration.

Figure 32 is the layout recommendation for that configuration. The schematic in figure 32 which labels the resistors Rf and Rg is the most common way to refer to those resistors if you are comparing with other texts (the feedback resistor from ground to inverting input, and the grounded resistor from inverting input to ground or reference voltage).

Thanks, I will look into that again, now nowing what to look for.

Are dB conversions familiar for you, or do you need assistance determining the gain ratio and appropriate resistors?

I know the concept, but don't know how to calculate the right values, so help would be much appreciated.

Are you sure the OPA134 is dead? Were you being literal in your earlier post that it "lit up a cigar," it literally glowed red?

Smoke came out, also I can no longer find it on my desk, I'll have a new one comming, I need to get some small bipolar 10uF caps for the 1646 anyway.

The OPA134 family is known for low distortion and clean sound, should be good for a clean preamp.
Also available as a dual in case you want to buffer the output for your pedals. Probably you will have to attenuate the signal if you are using typical guitar pedals, they would expect lower level. Something like the output of the first amp into the input of the second amp as a unity gain follower, which drives a resistor voltage divider to give the output level and impedance you want.

Yes, I was allready wondering that by creating quite a bit of gain my pedal outputstage would become too hot for the pedals if I would tap in front of the 1646, using the dual version for a separate FX output with the right impedance is a very good idea, I like it.

Thanks for sharing your thoughts on this.