Author Topic: Acceptable Noise Level - Suitcase Model  (Read 331 times)

Offline frenchji

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Acceptable Noise Level - Suitcase Model
« on: July 29, 2021, 01:52:01 PM »
I'm working on restoring a '75 Rhodes suitcase with a Peterson preamp.  I recently had the preamp, amps, and power supply worked on and recapped.  Got everything wired up and plugged in and everything seems to be working and it sounds amazing!

I'm curious about the noise level, and what is an acceptable level though.   

Should I expect some noise, similar to a guitar amplifier, or should it be dead quiet?  For reference, when I take the harp into a DI (bypassing preamp), It's absolutely quiet with no discernable noise.  When outputting into the suitcase speakers, I'm getting a little bit of buzz, similar to unplugging an electric guitar from an amplifier.  The "buzzing" or "hum" is consistent in level regardless of eq and volume.  A pair of the speakers also happen to be brand new, so I think I can eliminate the speakers from the equation.  The noise remains when unplugging the harp from the preamp, so I don't believe this to be related to the harp.  There is no noise when disengaging the preamp by unplugging the 4-pin cable.  This leads me to believe it's within the preamp?  That being said, there's little to no buzzing / hum on the headphone out and external amp out.

Again, the noise is an acceptable level, so it's possible I'm being a little too "OCD".

Below is a link to an audio file of the hum or buzzing I'm getting.

https://drive.google.com/file/d/1Ldcmc_OrvR2LdLSYYZBexQGjb8TM_w8L/view?usp=sharing
« Last Edit: August 02, 2021, 11:45:19 AM by frenchji »

Offline Tines&Reeds

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Re: Acceptable Noise Level - Suitcase Model
« Reply #1 on: July 29, 2021, 03:50:31 PM »
Sounds normal to me.

The Janus preamp has a gain of 22 in it's first stage which is huge. Since the preamp doesn't have any LPF at the output stage (like the Peterson Preamp), you amplify a lot of noise..
I've attached the frequency response of the Janus preamp for reference as you see it has its peak frequency at 11kHz with about 22db boost. The Rhodes doesn't have much to amplify in that range. So you are amplifying just noise....that's what you hear!
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Offline frenchji

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Re: Acceptable Noise Level - Suitcase Model
« Reply #2 on: July 29, 2021, 04:17:40 PM »
Sounds normal to me.

The Janus preamp has a gain of 22 in it's first stage which is huge. Since the preamp doesn't have any LPF at the output stage (like the Peterson Preamp), you amplify a lot of noise..
I've attached the frequency response of the Janus preamp for reference as you see it has its peak frequency at 11kHz with about 22db boost. The Rhodes doesn't have much to amplify in that range. So you are amplifying just noise....that's what you hear!

I should've mentioned I have a Peterson preamp, which I imagine is similar if not worse than the Janus in regards to noise?

What do the color bars represent?  The adjustment of Bass and Treble?  I'm assuming one of those lines represents the Rhodes "base" freq response?

Regardless, this was helpful, thanks!!!

Offline Tines&Reeds

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Re: Acceptable Noise Level - Suitcase Model
« Reply #3 on: July 30, 2021, 03:59:42 PM »
Yes. The lines represent the EQ settings in 10% steps. The violet line is with EQ flat.

The Peterson is quite different and has a built in fixed LPF, which kills a lot of noise. See attachment for a simulation of the Peterson (only treble boost is shown).

As you can see it has a smaller boost than the Janus of only 12-13db with the peak being at 5kHz. At 10kHz you only have +5db while the Janus has +22db. This is due to the LPF which is implemented in the Peterson preamp.
« Last Edit: July 30, 2021, 04:02:54 PM by Tines&Reeds »
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Offline The Real MC

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Re: Acceptable Noise Level - Suitcase Model
« Reply #4 on: July 30, 2021, 08:40:48 PM »
The pickups on the Rhodes are very sensitive to loading.  The input impedance of any preamp is crucial to noise and tone.

When I got my sparkletop Rhodes, none of the electronics were intact - no preamp, no power amp in the suitcase bottom.  So I experimented with DIs plugged right into the harp.  I had read that a 1Mohm impedance was ideal for that signal chain. 

What I found was that the noise and piano tone varied with different input impedances.  As input impedance goes lower the noise is louder and the tone is negatively affected.  Even my DI with 1Mohm input impedance was not free of noise.

But when I tried a Countryman Type 10 DI with 10Mohm input impedance, the noise was completely gone and I could get a nice full frequency tone.  I got both the fusion bark and bell tone.

I can't guarantee this will work with a '75 Rhodes.  They varied the pickup design over the years.  When I got my sparkletop I also had a '76 stage piano.  I tried the "magic chain" on the '76, and could not get it to sound as good as the sparkletop.  The pickups on the '76 are different than the ones on sparkletop pianos.

The Peterson preamp has a FET input for high input impedance, but to my ears the Peterson (I have a surplus one) is not as effective as the Countryman DI.  I have no idea of the topology of the Countryman as the components are potted in epoxy.  The built in LPF of the Peterson serves to cut noise but at the expense of the bell tone.

Also consider that Rhodes put really crappy speakers on the suitcase bottom.  Since my piano did not have any speakers in the suitcase bottom I experimented with others.  You can get a big improvement in tone using guitar speakers like Celestion 75w like those used in Marshall cabinets (why should guitar players have all the fun?).  That's what I installed in my suitcase cabinet.  I got awesome tone with Celestion alnico speakers like the G12s or "alnico blues" but they weren't hefty enough to handle the wattage.  Weber makes higher wattage alnico speakers but I haven't tried them.

You gotta experiment... your mileage will vary.

Offline frenchji

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Re: Acceptable Noise Level - Suitcase Model
« Reply #5 on: August 02, 2021, 11:24:37 AM »
Yes. The lines represent the EQ settings in 10% steps. The violet line is with EQ flat.

The Peterson is quite different and has a built in fixed LPF, which kills a lot of noise. See attachment for a simulation of the Peterson (only treble boost is shown).

As you can see it has a smaller boost than the Janus of only 12-13db with the peak being at 5kHz. At 10kHz you only have +5db while the Janus has +22db. This is due to the LPF which is implemented in the Peterson preamp.

I see.

I actually did a little more digging and there is some "hiss" in the high end I'm hearing but there's also a lower "buzzing" noise.  I was confused because my neighbor was mowing the lawn and the lawnmower seemed to mask the real culprit of the noise!  Almost sounds like when an electric guitar isn't plugged in all the way.  The "buzzing" is a consistent level regardless of eq and volume controls.  Not sure how I confused this with a higher freq noise the first time.

I also tested a few more scenarios.  I unplugged the preamp from the harp with no luck, still have buzzing.  Ran the external amp outs into a tube amp and that did clean up the noise considerably.  The headphone out has no to little buzzing.  I cleaned and tested all my contacts, I believe them to be functioning perfectly.  Buzzing stops when the 4-pin cable is disengaged so I still believe the culprit is the preamp.

The preamp was re-capped, so my first though was maybe the traces on the circuit board need to be cleaned up, since it almost sounds like a connection issue.  That being said, headphones and external amp out sound fine so I'm not sure this makes sense. 

At this point, I'm suspecting this is normal operation after all?  If anyone else can confirm there is a consistent low freq buzzing sound?

Offline frenchji

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Re: Acceptable Noise Level - Suitcase Model
« Reply #6 on: August 02, 2021, 11:31:45 AM »
The pickups on the Rhodes are very sensitive to loading.  The input impedance of any preamp is crucial to noise and tone.

When I got my sparkletop Rhodes, none of the electronics were intact - no preamp, no power amp in the suitcase bottom.  So I experimented with DIs plugged right into the harp.  I had read that a 1Mohm impedance was ideal for that signal chain. 

What I found was that the noise and piano tone varied with different input impedances.  As input impedance goes lower the noise is louder and the tone is negatively affected.  Even my DI with 1Mohm input impedance was not free of noise.

But when I tried a Countryman Type 10 DI with 10Mohm input impedance, the noise was completely gone and I could get a nice full frequency tone.  I got both the fusion bark and bell tone.

I can't guarantee this will work with a '75 Rhodes.  They varied the pickup design over the years.  When I got my sparkletop I also had a '76 stage piano.  I tried the "magic chain" on the '76, and could not get it to sound as good as the sparkletop.  The pickups on the '76 are different than the ones on sparkletop pianos.

The Peterson preamp has a FET input for high input impedance, but to my ears the Peterson (I have a surplus one) is not as effective as the Countryman DI.  I have no idea of the topology of the Countryman as the components are potted in epoxy.  The built in LPF of the Peterson serves to cut noise but at the expense of the bell tone.

Also consider that Rhodes put really crappy speakers on the suitcase bottom.  Since my piano did not have any speakers in the suitcase bottom I experimented with others.  You can get a big improvement in tone using guitar speakers like Celestion 75w like those used in Marshall cabinets (why should guitar players have all the fun?).  That's what I installed in my suitcase cabinet.  I got awesome tone with Celestion alnico speakers like the G12s or "alnico blues" but they weren't hefty enough to handle the wattage.  Weber makes higher wattage alnico speakers but I haven't tried them.

You gotta experiment... your mileage will vary.

DI into my interface is dead quiet and I think it sounds fantastic.  So if it's a loading issue, there's something wrong with the preamp I have, which is most likely true if I do have a problem.

One my amps blew at some point in time and took the connected speakers with it!  Half of my speakers are brand new, Eminence 1258 which are rated at 75w each.  Based off of some vintage Jensen speakers.  These sound fantastic and mix well with the vintage Rhodes speakers.  The noise is a little better, lower level, with the new speakers but it's still annoying.