PRR Vari-Mu / SlowBlow mic pre

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

PRR

Well-known member
Joined
Jan 30, 2010
Messages
11,143
Location
Maine USA
> the board for SlowBlow's mic pre

You have heater current flowing through signal grounds, right? Very naughty.

> planned PSU for the mic pre's B+ rail;

I sure hope there is a power transformer in there. It looks like a hot-chassis power supply, which is deadly. But given a power transformer, why not use a 230V winding and a plain bridge rectifier?
 
BTW: my limiter might be an OK mike preamp too, with a few changes.

Input transformer 500:50K Center-tapped, loaded in a pair of 100K resistors (or transformer makers suggested network). Problem is that there are few CT transformers with this high an impedance ratio.

Plate transfomer also 500:50K (does not have to be CT), load the secondary with about 100K.

Change the 10K(?) feedback resistor on the 5532 to a 100K audio taper pot.

Result is a maximum gain around 70dB, minimum gain around 30dB.

If you really need a 20dB pad with a conventional amp, you may still need it with this one.
 
> pin 5 is ground...

"Ground" isn't a bottomless hole you can dump crap into.

You are running heater power through several inches of PCB trace. And the tube cathode is connected to that trace. As an easy example: if the trace is 1 ohm, and the heater power had 1 Amp of audible ripple, the cathode would bounce up and down 1 Volt. In real life it may be 0.1 ohms and 0.1 Amps, "only" 0.010 volts of ripple being injected into the signal reference. At the input tube, the signal may be around 0.010V, so the ripple is as loud as the signal. Even if you get the PCB trace resistance down to 0.01 ohms, and the heater ripple to 0.001 Amps (0.3% voltage ripple on a 0.3A heater), the ripple is louder than tube hiss.

The tube plate power generally has to flow through signal reference ground. The heater power never does, and never should.

You have plenty of spare contacts in the connector. Have separate signal and heater "grounds". There isn't any great reason for the heater circuit to be hard-grounded, but if you do, do it at the power supplies.
 
[quote author="PRR"]"Ground" isn't a bottomless hole you can dump crap into. [/quote]

At least not until the zero-impedance conductor is invented :wink:

Connect the grounds of your various sub-circuits (e.g., heater, B-/signal, chassis/shields) together at one point only in the entire system, and you'll stay out of trouble most of the time.

Butta, take a look at this recommend system wiring diagram from an old Langevin catalog.
Page 1
Page 2

And here's a schematic of the 5116B preamp/line amp showing its pinout.
5116B
 

Latest posts

Back
Top