Neurochrome Hybrid Preamp Build

Emmett

Member
I am finally finishing version #2 of my DIY Neurochrome Preamp. The original version included the following:
2 - Neurochrome Unibuffers, one for Output 1 and one for Output 2 (sub-woofer)
1 - Neurchrome Input Selector Relay Board and associated Selector and Volume Control which functioned as a master volume.
1 - Neurochrome Preamp Power Supply
1 - Modushop 2U Slimline chassis

This preamp worked well except that I lost relays on the Input Selector board which I had to repair on two occasions. I assume this was caused by the lack of ESD production on the board. When I opened up the preamp for repairs the second time, I decided to make a few change in functionality. I wanted to add at least one XLR input pair and one XLR output pair. I also wanted to have a volume control for the subwoofer. I considered having two independent volume controls, one for each output port, but decided that what I really wanted was a main overall volume control and then a trim control for the sub-woofer output.

I decided on a hybrid approach. I retained the original Neurochrome Unibuffer for the main preamp and used the balanced output for Output #1. I added a Neurochrome Unibuffer on the front end to provide for the balanced XLR input. The output of this Unibuffer was routed to input no. 1 on my Input Relay Board. I then employed a P88 Preamp Board from Elliott Sound Products to drive the sub-woofer output (Output 2). Since this board provides the option for a volume/balance control between the two op amp stages on each channel, I was able to employ a master volume for both both Preamp 1 (main) and Preamp 2 (sub-woofer), and I was able to implement a trim volume (Volume 2) for the sub-woofer preamp (P88) (Output 2). I employed 1% film resistors that I closely matched on ESP 88 board along with LM4562 op amps. I also added a Elliott Sound Products P06 RIAA equalization board to provide phono support. The final change to my preamp was to replace the original version #1 Input Selector Relay Board with the new version #2 Neurochrome Input Selector Board which employs enhanced ESD protection..

The preamp is now in test operation. I'm using it with a DIY Audio Aleph J power amp that I built during the pandemic and a Polk Sub Woofer. The main speakers are a pair of vintage Magnapans. It all sounds great! It's working well except for noise in the P06 RIAA Phono Equalization board, which I assume is associated with pin connectors I'm using on the board. I expect that direct soldering of these inputs will solve this problem We'll see. However, at this point, I'm very pleased with the results.
 

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That's a pretty nice project there! Well done.

It's working well except for noise in the P06 RIAA Phono Equalization board, which I assume is associated with pin connectors I'm using on the board. I expect that direct soldering of these inputs will solve this problem We'll see. However, at this point, I'm very pleased with the results.
I'm a little bit skeptical of that explanation. Just because I've never seen connectors used at line level make much of a difference. Then again, the output of a phono cartridge is pretty small, so maybe... Definitely keep us posted on that.

I like that you have the RIAA stage and the power supply as far apart as possible. That's a good start.

Is the noise present even with the input to the phono stage shorted (i.e., RCA centre connected to its shell)?

Tom
 
I hear yeah, and I’m inclined to agree with you. I didn’t mention that I was using an old turntable from 1978 that hasn’t seen action in 30 years. I dusted it off, brushed off the stylus, and gave it a try. I was amazed it even turned on. There was sound but badly garbled. Needless to say I have trouble shooting opportunities here:) Stay tuned…
 
Some progress today. I checked the turntable on another system in the house. It worked fine, so that question was answered. I pulled the P06 RIAA board out of the preamp and examined it for soldering problems. I found one questionable joint which I reworked. I also reseated the op amps and then direct soldered the power connections to the board. I left the signal connections as they were. At this point, I have to note that the power to the RIAA board is daisy chained from the main preamp board (Unibuff).

I hooked up the turntable and tried it again. This time I got a reasonable sounding audio signal but with a significant underlying 60 cycle hum. I then added a ground lug next to the phono inputs and tried it again with the grounding wire from the turntable connected to the lug. This time I got a pretty good sounding signal from the turntable but still with a low 60 cycle hum. Things are improved but not yet totally resolved.

I'm now at a loss as to my next step. All of the line inputs are absolutely quiet. The phone input is the only one with a hum problem. I'm wondering about my grounding approach through the phono input. I am currently running the two ground wires from the Right and Left input GND lugs to the GND connection of the RIAA board. They come together at that point.

I then route the RIAA outputs (right channel HOT and GND and left channel HOT and GND) separately to the Input Relay Board. The two grounds come together at that point. I believe that any grounding problem is originating between the Phono input and the input to the Relay Board, since all five of the other inputs work as they should.

Suggestions are welcome.
 
I'd ask Rod Elliott for his input on how to best connect the phono stage. It sounds like you're picking up 60 Hz at the input to the phono stage and the phono stage does a wonderful job of amplifying it. A typical RIAA stage has ~60 dB gain at LF so it doesn't take much hum pickup to be annoying in the speakers.

Tom
 
Well, I broke down and did some research on how the phono section should be wired. Most helpful was a schematic from the old Dynaco PAT4 I found online. Should have done this before I wired it the first time. Anyway, I had not fully shielded the phono wiring from the phono input to the RIAA board. I decided to rework that wiring so that it is now fully shielded up to the RIAA board input. Dead silence, no more hum! It sounds really good. Everything is working with no discernible noise. I still need to add some text next to the LEDs on the front panel. Otherwise, I think this project’s done:)
 
Fantastic! I hope you'll post pictures of the completed project.

Tom
 
Here's the completed preamplifier. It has six inputs including one balanced input pair and a phono input with RIAA equalization. The preamp has two independent outputs including one balanced output and one RCA. The RCA output is targeted as a sub-woofer output, but does deliver a full range output. As you can see in the photos, there are two different preamp boards employed. The preamp feeding the balanced output is a Neurochrome Universal Buffer (Unibuf), and the sub-woofer preamp is a Elliott Sound Products P88 Preamp Board. There is a reason for this approach. I wanted to have a master volume control over both output ports, and I wanted a slave volume control for the sub-woofer. In practice, it functions a lot like a loudness control and keeps me from having to reach behind my sub-woofer to adjust its level. The P88 board is set up for employing a volume and balance control between its first and second stage op amps. The signal coming from the master volume goes to both preamps, and the second volume control essentially raises or trims the sub-woofer level.

The other components employed in the preamp are a Elliott Sound Products P06 RIAA Board, a Neurochrome Universal Buffer servicing the balanced input, the Neurochrome Input Selector Board Ver. 2, and a Neurochrome Preamp Power Supply.

As mentioned in entries above, I encountered 60 cycle hum issues with I first fired up the phono section of the preamp. I looked through several forums for instruction on the right way to wire the phono input but didn't find what I was looking for. I did a search for old Dynaco Preamp schematics and used the PAT4 preamp schematic as a guide. I rewired the phono input to incorporate shielding from the inputs to the RIAA section, and the problem was solved. No hum.

I plan to add labels for the front panel LEDs and may change the designations on the volume controls. However, right now I want to listen to it and think about options for the next project. Cheers!
 

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I rewired the phono input to incorporate shielding from the inputs to the RIAA section, and the problem was solved. No hum.
Epic! Thanks for sharing the solution. Cheers!

Tom
 
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