How to handle positive chassis (adding in a RIAA pre in olde schoole Germanium reciever)

[Disco Volante]

The young man I mentor a little bit in tinkering has recapped this old Beomaster 900. It now works quite well by its own standards (6W of true Germanium power!).
We've managed to reduce the noise somewhat by removing some resistors and rewiring it a bit, also replaced a few Rs with metal film.
All that's lacking is a proper RIAA preamp. These machines are meant to be used with crystal pick-ups.

So, I had this old Velleman thing in the drawer, and adding in a 7815 to drive it on a piece of vero-board is easily accomplished.
Now, what to do about the positive chassis? Reverse the supply on the riaa-pre? If so, how do we avoid shorting the -24V via the input socket? Yes, we can separate signal ground from chassis, but what if someone plugs in a DIN-plug where shield and signal ground is connected?
Or am I barking up the wrong tree here?

Cheers, V

 

[Disco Volante]

A bit of reading later:
TI Single Supply Op Amp Design Techniques says
"Unless otherwise specified, all op amps circuits are single-supply circuits. The
single supply may be wired with the negative or positive lead connected to
ground, but as long as the supply polarity is correct, the wiring does not affect
circuit operation.
Use of a single-supply limits the polarity of the output voltage. When the supply
voltage (VCC) = 10 V, the output voltage is limited to the range 0 ≤ VOUT ≤ 10. This
limitation precludes negative output voltages when the circuit has a positive
supply voltage, but it does not preclude negative input voltages when the circuit
has a positive supply voltage. As long as the voltage on the op-amp input leads
does not become negative, the circuit can handle negative input voltages.
Beware of working with negative (positive) input voltages when the op amp is
powered from a positive (negative) supply because op-amp inputs are highly
susceptible to reverse voltage breakdown. Also, insure that all possible start-up
conditions do not reverse bias the op-amp inputs when the input and supply
voltage are opposite polarity."

So a 7918 negative regulator on the -24V supply should do it and just make sure the polarity is correct...

Cheers, V

 

[Voyager10]

You can do something like this:



There's no need to step down 24V to 15V, unless the capacitors have a very low voltage rating.

So

• Pin 6 is ("positive") ground
• Pin 5 goes to -24V
• Input and output grounds (1,3,8,10) are now pin 6.
• Coupling capacitors C6, C7, C8, C10 need to be reversed, i.e. +ve ends connected to the input/output pin.
• C1 needs to be across R3, not R1

 

[Disco Volante]

Thanks, that makes sense.
I was going to put a regulator in front to isolate the op-amp from the noise on the -24V rails.
Cheers, V!

 

[Voyager10]

OK, it looks like the unit already has a regulated -24V rail (the 2N555 transistor etc) so it shouldn't have too much ripple, but a second regulator wouldn't be harmful.

Probably the most important thing is finding the right place to connect the ground. RIAA amps have most gain at low frequencies so mains hum is the likely concern.

C1 is important for power supply noise rejection, generally the bigger value the better.

 

[Gold]

A true crystal pickup doesn’t need an RIAA preamp. Which is probably why it isn’t there. It is a constant amplitude transducer unlike MM and MC cartridges which are constant velocity. The constant amplitude ceramic cartridge needs no additional eq to have flat reproduction response.

 

[Gold]

A little OT but maybe somewhat relevant. Panasonic invented what they call a “strain gauge” pickup. It is a crystal cartridge. The difference is that the strain gauge cartridge Is biased by a current and made to work in voltage mode.

These devices are/were expensive. A current manufacturer of crystal strain gauge cartridges is https://www.sound-smith.com/welcome-soundsmith

These cartridges need a special preamp to provide the bias current and the RIAA EQ which is necessary in this case.

 

[Disco Volante]

Yeah, I've heard about the "strain gauge" system. Seems rather exotic...

 

[Mr. Moscode]

I made a Hybrid Tube-FET RIAA preamp called the "Super It" built by NYAudio Labs in the 80's. 57 db gain stock with volume and balance controls.
You may be able to find one used on Audiogon or ebay, etc. Otherwise there are a lot of opamp RIAA pres out there along with plenty of schematics if you want to build your own. Plug 'n play.
Xtal cartridges don't need RIAA. We had an Astatic ceramic one in our Garrard RC88 record changer. They are hard on vinyl so I don't recommend them.

 

[Gold]

The KA Electronics Phono Transfer System is a beast. Probably overkill for this.

You could do much worse than the NAD phono pre. It’s an excellent sounding phono pre using only a few transistors.

 

[Disco Volante]

Yeah, no shortage of interesting RIAA pres. I am acquainted with JC Morrison, who's engineered an interesting phono preamp, the SIren Song...
For this specific project we wanted to keep it small and reasonably simple, to be built into a vintage reciever.

Maybe the next phono pre will be tube equipped. Those 5654's are eyeballing me from the shelf...

I mainly know the crystal pickups from the generally awful little 7" record players that were commonly available when I was a boy. Not something I'd subject my record-collection to.

Cheers

 

[Disco Volante]

While we're veering a bit anyway:
Is there really any advantage to using a 12" arm over a 9"?
Is it true what they say that the 12" arm better replicates the geometry of the lathe?

Never actually owned one of those loong arms.

Cheers

 

[Mr. Moscode]

While we're veering a bit anyway:
Is there really any advantage to using a 12" arm over a 9"?
Is it true what they say that the 12" arm better replicates the geometry of the lathe?

Never actually owned one of those loong arms.

Cheers

No pivoted arm replicates the cutter lathe which goes straight across the disk. The longer has a larger radius arc, a little closer to a straight line, so cartridge angle error will be less. It will also have more mass which has its pluses and minuses. If you use a quality cartridge you want a good TT. This doesn't sound like a hi budget project (but I could be wrong) so good enough may be good enough.

 

[Gold]

What Mr. Moscode said. The geometry is better for tracing a straight line from the outside to the inside diameter.

The sound of a record at the inner diameter with a 12” arm is greatly improved. I would go so far as to say that if you are doing record transfers for money and you aren’t using a 12” arm you are doing it wrong.

There are some great deals these days on Japanese arms from the 80’s. Prices have been coming down. IMO the Japanese playback equipment from the 80’s is the best it gets.