Idea for a dedicated Rhodes preamp, need help !

[ccaudle]

I get is a very loud signal / noise as soon as I switch it on..

Do you still have C3 and C4 as power supply bypass? Those should be as close to the power pins as practical.

C2 in the schematic by ccaudle (wich I am using as my lead) in post #73, do I need it ?

Probably not with 2134, those are pretty stable. And when needed they tend to be in the small nF range, not uF.

 

[PermO]

Ah, thanks for this !

Yes I have two small ceramic bypass caps at the + and - 15V at both the IC's .

It seems to work fine now, I think it was the combination of the unterminated 10M input and the high gain that made it "fly off the handle" at first power up (I did not have it connected to anything but the VU that is connected to the output of the pre amp box went straight into the right corner)

I am using 2.2K for R3 in my test setup now and the guitar sounds really nice this way.

I'll do some more experimenting with R3 to see what range I need on my rotary switch.

 

[Brian Roth]

It is possible that the circuit was going into oscillation which can sound like a lot of "hash".

Bri

 

[abbey road d enfer]

Another thing I can't figure out is the C2 in the schematic by ccaudle (wich I am using as my lead) in post #73, do I need it ? is it good practise to put it in anyway, and what value are we talking about, is this a nF thingy or more than 1uF ?

It depends on the value of R2.
This capacitor is there for stability. If there is no parasitic oscillation, just delete it.
However, it can also be used to tame HF, and again it depends on the value of R2.
You will se that HF attenuation is also gain dependant.
You may try different values and tune by ear.
1uF is probably way too much.

 

[PermO]

To start with, thanks to all who contributed ! ... this circuit sounds really good !

After some more experimenting, ..So what have we learned today ?

Well, my 12 pos Lorlin switch did not have 12 positions at all, but 6, because there were two rotating contact thingies inside there was always two resisitors selected instead of one wich resulted in a very strange behaviour of my gain switch.
I fixed that by opening the Lorlin switch and remove one of the contact lugs.
But my gain stage was still acting weird..
I put in C3 from the schematic in post #73, so the gain selector connects to ground via a capacitor, only knowing it is there for DC blocking I decided to put a 0.68uF filmcap in...
That resulted in some sort of envelope / tonecontrol thing that you don't want, so I bypassed the cap and it is no longer acting funny.
I will try with a really small capacitor and see how it behaves.

I think the initial "run away" behaviour was caused by a bad connection on the plastic proto board, it's the first time I used one and I allready decided I don't like these things, components sit wobbly and connections can not really be trusted, my initial thought was to just keep the circuit on the proto board but no, components need to be soldered.

That oscillation has not not happened again, also with nothing plugged in at the highest gain setting.

It has got a ton of gain, sounds really good and so far is surprisingly quiet, with everything still sitting on the proto board and the top cover off (no audible noise or hum)

Switching does not result in loud clicks and pops, there's a little bit of a click sometimes when switching but it is not problematic at all, probably not good enough for a commercial high end product.
Don't know if these cheap Lorlin's are make before break, but I had a couple of these in my drawer so I used one.

I'll play around with it some more and transfer the circuit to two little circuit boards, one for the balanced output driver and one for the pre-amp section, so I can easyly swap the circuit if I want to try something else at some point.

I'll post a full schematic including my PSU when finnished, and some audio clips ofcourse.

 

[PermO]

Ah, drinking some coffee and playing a bit I discovered something...

The unit does self oscilate when nothing is plugged in at the higher gain settings.
With just the guitar cable plugged in, and no guitar, so still an open connection it does not do that...

"Changing the antenna" makes it go away.

So I figured a 100R series resistor on the input might fix this ?

I'm looking at guitar preamps in the small audio signal design book and it says the series resistor with the cap form a low pass EMC filter.

 

[abbey road d enfer]

The unit does self oscilate when nothing is plugged in at the higher gain settings.
With just the guitar cable plugged in, and no guitar, so still an open connection it does not do that...

"Changing the antenna" makes it go away.

It's the added capacitance of the cable that does stop the oscillation. A capacitor at the input would probably do the same.

So I figured a 100R series resistor on the input might fix this ?

I doubt it very much.

I'm looking at guitar preamps in the small audio signal design book and it says the series resistor with the cap form a low pass EMC filter.

According to the schemo, there is no cap, so actually the filter would be constituted of the 100R resistor and the parasitic capacitance of the stage, just a few pF, for a cut-off frequency of several 100 MHz.
Simply, never leave the input unloaded.

 

[PermO]

It's the added capacitance of the cable that does stop the oscillation. A capacitor at the input would probably do the same.

Ah, would a really small cap on the input mess with the sound ? ...would there be a reason not to put a little cap there ?

I doubt it very much.

Correct, I tried, it doesn't do a damn thing

According to the schemo, there is no cap, so actually the filter would be constituted of the 100R resistor and the parasitic capacitance of the stage, just a few pF, for a cut-off frequency of several 100 MHz.

C1 input serial capacitor, 1uF

Simply, never leave the input unloaded.

Yes I agree, I can live with this, it's not a commercial product... I really like how it sounds, I don't wan't to change that.

 

[abbey road d enfer]

Ah, would a really small cap on the input mess with the sound ?

It may, or may not. That would depend on the actual impedance of the source.

...would there be a reason not to put a little cap there ?

Considering the chosen nominal input impeda,nce of your circuit, 10 Megohm, it makes it suitable for high impedance sources, and in that case, a cap on the input will probably interfere with the frequency response.

C1 input serial capacitor, 1uF

A series 100R resistor would not react with the series capacitor to form a low-pass filter. It reacts with a shunt capacitor, which there is none, except some parasitics..

 

[PermO]

Oh, wait.... it's much more simple I guess...

Just use a switching input jack that connects the input to ground when no jack is plugged in, that should stop the oscillations from occuring.

I don't have that now, but I'll drop one in my basket with my next order.

 

[PermO]

Soldered on circuit boards, with the top cover on.... no more spontaneous oscillation.

I like it.

 

[ccaudle]

Soldered on circuit boards, with the top cover on.... no more spontaneous oscillation.
View attachment 92787
I like it.

Have you tried it with your Rhodes yet? I only saw mention of trying with a guitar so far.

 

[PermO]

Yes, but the guitar was easyer to move to my workbench

I have been playing the unit for some hours now, in my studio setup, bass, guitar and Rhodes piano and I really like how it sounds / performs, this is a high quality instrument pre.

I've done some listening and pre swapping, a passive DI is no competition for this thing, at all, different planet.

Compared to the 600K jfet input on my BLA B173 "Neve clone" well, that sounds like cardboard next to the proto preamp.

Compared to my Neve RNDI stereo (into mic pre of my mixer), and my DIY Alembic bass preamp clone (12ax7 / 12au7 / Edcor 15K/600Ohm ) it is becomming a choice, they all sound slightly different but none of them is the clear winner.
Though I think the RNDI and Alembic add some color, the RNDI has this "sparkle on top" and I got very close to that sound by driving a transformer from the Proto Preamp.

On bass the Proto Preamp is the clear winner, in my opinion real low end and subharmonic content do not go well with transformers, solid state is much more tight and punchy.

I've been experimenting with C1 the only component that's in the signal besides the IC's ... the circuit brings out so much detail it is like putting a magnifying glass on C1 I started with 1uF MKC thinking that should be way oversized with this input impedance, I went as far as 3,3uF MKC and that brought out the really low end "thump" playing the E string.
The MKC can be a bit grainy so I put a Siemens styro 200nF across and all was nice.

Calculating this,... going from 1uF to 3.3uF probably makes no sense at all, I know, but if I can hear it, I settle with that.

I put 6.8nF for C2, (R2 = 10K) I started with larger value and brought it down where I could no longer clearly hear it having an effect, maybe it does sit there doing nothing at all, I don't know. (I did not have any smaller ones) and i don't know how to calculate the effect of this capacitor.

I'm drawing a full schematic of everything that's in the box and I hope to record some clips this weekend if I am not too busy.

I'll be visiting a friend who likes to see how this performs on the testbench, so I will get a frequency response plot and noise figures.

 

[abbey road d enfer]

I put 6.8nF for C2, (R2 = 10K) I started with larger value and brought it down where I could no longer clearly hear it having an effect, maybe it does sit there doing nothing at all, I don't know. (I did not have any smaller ones) and i don't know how to calculate the effect of this capacitor.

You must be aware that the effect of this cap is a shelf that goes from the dialled gain at LF to unity at HF. Actually unity gain is probably achieved at ultrasonic frequencies. The response is different for every gain position.

 

[PermO]

I just ran a tonegen sweeping the input and watching my VU ...yes the 6.8nF is still filtering... about 7dB @20KHz at half gain...

It got better with smaller sized caps and I stopped at my smallest one and left it there... but it needs to go, I don't want any filtering going on, I have equalizers for that.

I draw my schematic with pencil, so it's just as easy to remove from the schematic as it is from the unit.

 

[PermO]

"All caps are bad !" "Defund the caps !" ... it was not doing much harm, but yes, without the cap it's better.
Here's my drawings;

 

[abbey road d enfer]

"All caps are bad !" "Defund the caps !" ... it was not doing much harm, but yes, without the cap it's better.
Here's my drawings;
View attachment 92806

I believe changing settings of S2 or S3 results in clicks, unless they are MBB (make-before-break).
I think R1 should always be connected to ground, then R17 changed to 4.7Meg.
Similarly, R6 should always be grounded, and R7, 8 and 9 modified accordingly.

 

[PermO]

Yes I agree that would be a much more elegant solution, thanks, I will apply that in the final version.

I don't have any nasty clicks on the gain switch, sometimes there's a a little bit of switch noise, but it is below -20dB and does not show on my mixer VU.
I noticed the contact lug in the Lorlin switches is rather large, I think it touches the next contact before it leaves the previous.

The input impedance switch does result in audible clicking, and I see why now.

 

[PermO]

I've been playing with this calculator;

http://sim.okawa-denshi.jp/en/CRlowkeisan.htm
To look at the effect of C1 at the different input impedance settings.
There's more going on than "just frequency response" if you look at phase behaviour at subharmonic frequencies.

I think my choice for a 3.3uF is not that crazy, it will provide a flat response at all impedance settings with no phase issues in the subharmonic band.

If I would stick with my first 0.1uF cap that would give a cutoff @80Hz at the 20K input setting, that's is not going to sound fine on your Moog bass synthesizer, phase issues would be present onwards to 200Hz...

The 1uF cap would be fine, but the 3.3uF gives me the security that there is no impact on frequency response and subharmonic phase behaviour at all... and I like that, it gives me piece of mind.

Also found this interesting bit in another topic;

... into hi-Z op amp which is used to drive THAT part properly, which is prob driving a 1:1 output

So I'm not the only one who uses this sequence "that doesn't make sense" ..at least my transformer can be bypassed.
I think it's nice to have the option, as I had the transformer for free anyway, it introduces a little bit of "transformer magic sparkle" in the high frequency range, wich is nice on guitar. On bass I prefer it bypassed.

 

[abbey road d enfer]

There's more going on than "just frequency response" if you look at phase behaviour at subharmonic frequencies.

Phase response and frequency response are just two sides of the same coin, at least in a Minimum Phase system, which is the case here.
However, the human sensory system is only sensitive to the frequency response; occurences of phase anomalies being detected by audition have always shown that it was related to non-linearities of the transmission chain, which result in frequency response or peak factor modifications.