DIY 12AU7 Compressor Troubleshooting

[PRR]

With C4 un-jumpered, that voltage across C4 should be "zero" for no or very-small signal input, going to Negative 5V and 10V for very large signals.

I don't like the 10KHz input. Can you try 2KHz?

Can you try actual music? Dynamic music, not the squashed stuff on radio and many commercial recordings. This compressor should approximate the "squash" of radio station signal processing.

 

[somorastik]

PRR, thank you for reply I really appreciate it.

I will investigate on the "zero " voltage you describe.

I will try with 2kHz, but the signal voltage drops significantly, too tiny transformers... I am starting to think they are not suitable for this job, but they are described as audio transformers by the seller:

Transformer type : low frequency
Transformation ratio 1:1
Primary winding resistance 4.6Ω
Secondary winding resistance 4.6Ω
Secondary winding impedance 280Ω
Primary winding diameter 180µm
Secondary winding wire diameter 180µm

I tried this configuration: Dynamic Mic (XLR)->Preamp (symetrical XLR output)->Compressor (symetrical XLR input)->Scope (single end output of compressor)
I put my android phone on loud with signal generator and some sine wave set under the microphone.

The wave was deformed, the pots had veeery little effect on the signal amplitude, the same as with signal generator.

btw.: Is there any problem with using the input for single ended (unbalanced input), I connect 1 and2 of the XLR input to the Ground and use 3 as HOT signal wire.

I will try dynamic music today.
I have very little time to DIY, only in the evening, so yeasterday I DIYed till midnight :-D I usualy quit on midnight or when doing tube amps when high voltage kicks me, or when I set off a breaker coupla times.  8)

 

[radardoug]

Those transformers are completely useless for a serious audio project. You need to change them for a start.
First thing, get a job, earn lots of money, and then do some DIY. You have your life backwards!

 

[PRR]

> described as audio transformers

That covers many things.

> Secondary winding resistance  4.6 Ohms
> Secondary winding impedance  280 Ohms

The specified transformer was 600:600. 280 is a bit shy.

A "good" audio transformer will have resistance/impedance around 1/10 or 1/20. Much less, there are not enough turns to handle audio bass, maybe not even midrange. As your tests show.

It may be quite difficult to get "useful" operation this way.

The originally specced transformers were "Automotive Ground Loop Isolator" used in car-sound installation, such as RadioShack #: 270-054, $14.99 bought two transformers in a little can with phono leads. Radio Shed has changed the part number (why!?) to 2700054, and it is still $14.99, AND there are four in-stock 9 miles from me. (Or buy on-line, but buy two and add a resistor so you get over the $30 free-ship threshold.)

 

[PRR]

OK, SV1-500 is made in India and has a datasheet at Schukat.

Inductance:
@ 50 Hz/1V / @ 1 kHz/1V
N1 = 34 mH / = 22 mH typ.
N2 = 34 mH / = 22 mH typ.

34mH is 0.034H.

Assume 100Hz?

0.034H * 2 * 3.14 * 100Hz = 22 Ohms impedance at 100Hz.

Drive impedance from my design's plates is 900 Ohms. Simple extrapolation suggests a 4,100Hz "bass cut". That's so far above 50Hz that we should use the 1KHz inductance: 0.022H. More like 6KHz.

These irons are just unsuitable. They may work at very low impedance (30 Ohms) for limited-bass applications. They don't even seem good for that. They appear to be modeled on an old-old Japanese mini-transformer series, but don't seem to work as good.

That Radio Shack "auto ground isolator" is just an example. The same/similar product can be found anyplace people put booster-amps on car radios.

 

[PRR]

Here's my 2003 notes in a ZIP file. Much of it is in the Comp5.html web-page, but there's a few stray bits.

I had to re-name the ZIP file with a TXT extension to get past the forum upload. Save As (not direct click) to your machine, rename by stripping off the TXT extension, then it should un-ZIP.

 

[saint gillis]

Hello guys, you use a 600:600 transformer as TX_out ?
Isn't that supposed to be weak to be driven by the tubes' plates?

 

[PRR]

1) the design goal was "inexpensive"
2) if it "works" but the bass is not good, then buy a better transformer.
3) there is a LOT of gain after the transformer, just to get to a -10dBv level. How much signal is on the transformer?

 

[saint gillis]

3) there is a LOT of gain after the transformer, just to get to a -10dBv level. How much signal is on the transformer?

I don't know how tube gain stages are calculated, but something like 20db of loss on TX-out transformer?

 

[RuudNL]

The link in the first post shows a very cheap and small transformer, that looks like a 'modem' transformer to me.
This is certainly not what you want to use if you expect some reasonable audio quality.
The first thing I would check is if you get a DC voltage over C1 with R39 full open. (Of course with audio input!)
I would at least expect a couple of volts here.

 

[saint gillis]

I know that, just willing to learn a bit more about transformers loads and sources and rated impedances.
I mean, right now I'm not building this device..

 

[saint gillis]

By the way, is it correct that TX_in is less an issue to load than TX_out ?

 

[saint gillis]

I guess an ouput transformer might work as tx_in if driven by a low inpedance source like an op amp before tx_in inside the device... But a 1:1 line output transformer is usually intended to be driven by a very low impedance source, so I guess it is not the best choice for tx_out... the gurus might correct me if I'm wrong

 

[saint gillis]

Any direction on this and maybe some reason the wattage is important???

I guess an output transformer has a low DC resistance and not so much inductance so the triodes have to deliver a lot of watts without managing to drive correctly tx_out ?