BHK Phono preamplifier


#1

For those wondering about the upcoming BHK phono preamplifier, I include a photo off Bascom’s bench.

I love this photo. Bascom and I design the same way, with porcupine wires and push board for components. It’s a thing of beauty as you can see, though some might be horrified at the mess - just think of it as genius at work.

In the back with the chassis on its side is where two tube sockets can be connected to the breadboard in front of it, and behind that the chassis is the heater supply (bottom) and a high voltage supply on top. In the foreground is a simple solid stage phono preamp breadboard Bascom uses for the RIAA EQ and its output stage for testing the tube and FET input stage. To the right are additional power supplies for that SS phono preamp and for circuitry on the tube breadboard.

Bascom's big challenge at the moment is lowering noise when using a tube/FET input stage.

Just thought you'd all enjoy a peek into the working of BHK as he moves forward with yet another design.

Phono-preamp-Kludge.JPG


#2

I guess the pill bottle is for headache medicine.4_gif

Very cool, thanks!


#3

Is the new phono preamp going to include an A to D converter for ripping vinyl like the NuWave Phono preamp?


#4

No, it’s purely analog.


#5
Paul McGowan said ... I love this photo.

Me too. When I was an engineer still in my 20's my boss tasked me with designing a simple circuit. I remember my own workbench looking something like your photo. However, it didn't work. It didn't do remotely anything that even my sainted mother would construe as 'working'. It was the last time I ever tried my hand at electronics design :)

#6

That’s a shame - you’re so damned talented! Though I understand. I once took great pleasure in smashing to bits an entire power amp circuit board with a hammer so it would never cause me grief again.

Then I discovered the variac and life changed.


#7

Paul, in viewing the prototype I didn’t notice any tubes in the circuit. Perhaps it is too early in the design. Will the BHK phono preamp include tubes in the circuit? Thanks.

The tubes are on the top of that aluminum chassis and face away from view. And yes, it will have tubes in the input and output stages of the circuit. And like my previous BHK designs, will also have MSFETs and JFETs where needed.

BHK


#8

Great news, you’re also working on a phono amp, so even if it means a few k$ more for add. cabling compared to a one box solution, I’ll possibly also have an extended PSA chain sometimes :wink: Probably instead of a headphone amp a phono stage wouldn’t have fitted into the preamp anyway.

There are three questions I’d like to ask at the moment:

  • will it have tubes (as tubesrule asked) and balanced inputs?
  • often, like my current one, preamps or phono amps have a separated power supply (even if in a smaller, less costly chassis). My understanding is, this helps avoiding interferences. You didn't choose so with your preamp and I wonder if you'd do with the phono amp. Is it a matter of cost vs. effect?
  • do you already have design goals like a very flexible design in terms of adjustments like a Manley... or a reduced, no-nonsence design that fit's all?

#9

I hope the stereo/mono switch isn’t overlooked.


#10
tubesrule said Paul, in viewing the prototype I didn't notice any tubes in the circuit. Perhaps it is too early in the design. Will the BHK phono preamp include tubes in the circuit? Thanks.
If you look closely near the back of the bench you'll see an open chassis sitting on its side and facing us are two tube sockets (well, the wiring side). It might be hard to see.

But yes, by all means, there’s two tubes inside.


#11
bfotk said I hope the stereo/mono switch isn't overlooked.
Good point.

#12

A note on the experimental breadboard setup in the initial post on this subject, that is a one channel circuit, not a working stereo one.

I thought I would clarify the intentions for this phono preamp design. First it is to be a fully balanced circuit that would allow the cartridge to be floated off of signal ground - or not. There will be a tube differential amplifier first stage that may or may not be augmented with some FETs below it for lower noise. Following the input stage will be a passive RIAA equalizer that may be the full network or just the 2120 Hz rolloff part. If a partial equalization network, the output stage will have active feedback to generate the 500 -50 Hz bass boost. If the equalizer is the full network, the output stage is flat in frequency response. This describes the main signal circuitry and is setup for MM gain. For MC inputs, a stepup transformer of suitable quility will be used.

There are a lot of considerations in all of this. First is the issue of input noise. I am most familiar with the work of John Curl and James Bongiorno in achieving super low noise MC input stages. Personally, I am not all that worried about a little input noise if the sound is better. I once made a tube MM/MC phono preamp using very low noise 7788 tubes. The MM 2120 Hz HF rolloff was with a series 47K resistor and a suitable capacitor at the input of the tube. Heresy! All of those 10’s of K of noise resistance ! This also provided the essentially 47K loading for the cartridge up in the 20 kHz range. It was quiet enough to not be audible at the listening position at normal playing gain. I also did the same thing for MC use by a much lower impedance RC input network. That also worked pretty dammed good with the same circuit. So bottom line on this, this preamp circuit may not end up as the lowest noise circuit on the block but it going to sound dammed good and serve the music.

Aside from considerations of shielding a power transformer from hum radiation in the same chassis and issues of power supply regulation, the main issue for me in this initial circuit experimentation is the design of the second gain or output stage of the circuit. First is the matter of whether it is a bass boost equalizer or flat amplifier. The output devices in this proposed output amplifier are P channel MOSFETs that would be operated in the common source mode with attendant high output impedance. Overall output circuit feedback will reduce this to a sufficiently low value. Now if it is the flat response version where the RIAA equalization is all done in the interstage network between the input and output amplifier, all is well. However, if it is configured as the 500-50 Hz bass boost part of the RIAA equalization, the output impedance is going to rise as frequency goes down below 500 Hz because the amount of feedback is going to decrease. This will have the effect of the very low bass frequencies will rolloff more with low impedance loading on the output of the preamp. Also there will be different amount of input signals available to the output stage in each case. Less signal for the full interstage EQ and more for the 2120 HF rolloff case.

More on the circuit for the output amplifier. It is going to have a differential tube input stage. Unlike the first stage of this phono preamp and the tube stages of the BHK preamp and power amplifiers where the tube stages stand alone outside of any including feedback loops, this first stage of the output amplifier will be included in the circuit’s overall feedback loop. Servo circuits will keep the output phase to phase and each phase to ground close to 0 VDC.

This is enough for now and I realize it is too technical for some but not others. If there are good ideas to consider in this and perhaps alternative circuits to consider, chime in to contribute!

BHK


#13

Thanks a lot for this insight! Although I understand way too little about the technical details, it’s great to read and I wish I studied electronic engineering and I could participate more in detail.

I look forward to reading more about this project.


#14

Hi…

Almost one year since the last post from Paul…any news on BHK Phono Preamp?


#15

Or the speakers ?


#16

PCBs for the phono preamplifier are finally in process but not production. Darren made PCBs for Bascom so he can start building the prototype on a single board instead of the wired kludge he has now. So, still a work in progress but at least progress is being made.

Arnie’s speakers are finally moving forward and the drawings for extrusions and cabinet are done and at the manufacturer who is trying to get us a quote for the cabinets. Frustratingly slow. Hopefully within another month or two we can get cabinets and drivers for Arnie to start his magic.


#17

This is timely as I’m thinking of a phono pre upgrade. Paul, do you have any rough idea of price point and ETA?


#18

Paul

May I recommend getting some audio research Ref 2SE/3 phono preamps as a benchmark.

Not for sound quality :slight_smile: heck no. I trust Bascom and you have enough experience but in terms of the user interface.

The ability to change the gain using a remote control and change the loading on the cartridge via the remote is a godsent when trying to set up the cartridge right from your seat.

Having dual inputs, XLR inputs from the tonearm would also be nice.


#19

I doubt Bascom will have it finished and we release this calendar year. Look for 2018.


#20

Will the new phono preamp have a mono button?

There seems to be no practical way to play mono vinyl without changing to a mono cartridge. This is not a practical solution. One option is to use y cables to sum the right and left signals. I heard that this presents a problems since resistors should be introduced for optimal sound. It also is not practical. Back to the mono button… This would be the answer, it seems. Any comments on how to best play mono without changing to a mono cartridge?

Any update on the release of the new phono preamp?