BTDR-3 reverb module

[xtian]

I plugged the circuit into my amp, to real-world test it. Works very nicely. Very low noise floor--can't hear it over the hiss of the Bandmaster it's plugged into. Did A/B test--guitar thru circuit may be slightly gained up, slightly brighter, but not much. Pretty close to unity. Reverb sounds good. Very springy, lots of pre-delay (100ms?).

I bought three of the Rub-a-Dub Deluxe PCBs from 1776effects@gmail.com for $30 including shipping.

[JazzGuitarGimp]

While adding a large-enough capacitor in parallel with the feedback resistor _can_ be used to shape tone (high frequency roll-off), this cap is most often used to stabilize the opamp stage - to keep it from oscillating, especially in high-gain stages. A 100pF cap has an Xc of 80K-Ohms at 20KHz, and that number increases as the frequency goes down (800K-Ohms at 2KHz, 8M-Ohms at 200Hz). So it isn't going to affect your HF response. However, at 1MHz (about the beginning of where oscillations occur), the Xc is only 1,600 Ohms, which, in parallel with the feedback resistor, sets the gain of the stage to well less than 1.

So, short answer: the caps are there to stabilize the stage.

[xtian]

Awesome. Many thanks for your help.

[xtian]

I put my meter in-line, and see that the circuit is consuming 375mA. No wonder it sucks batteries dry quickly!

Clearly, we cannot just tap the HT winding to feed this hungry bastard.

This brings me to my THEORETICAL question: how do you go about choosing resistors for a voltage divider off B+, knowing you need ~400mA to supply your circuit? Let's say B+ is 400v, and you want to send 18v to your 15v regulators.

[Theashe]

375mA? Damn! And the datasheet said a maximum of 100mA. Would it be too much effort to buy a small PC mount transformer like a Hammond 229 series and put a small split-rail power supply on a perfboard inside the amp? I've attached a picture of one of the 6VA transformers I have as a reference for their size.

[xtian]

Ha! Now that you reminded me, I looked at the datasheet again. It says 60-100mA. Hmm.

I measured again, this time just between the +5v regulator and pin 1 on the BTDR, and I see 238mA.

[Theashe]

Whew! You'd have to draw all that current through your whole power supply if you wanted to get those voltages from your B+ string. You'd need a pretty fat PT and high power resistors to handle all that. If your PT has a fixed bias tap, maybe you could use that instead and save some VA.

Are the batteries in your multimeter good? I had some dying batteries giving me misleadingly high voltage readings a few months ago.

EDIT: I've been planning on putting a BTDR reverb unit in my next amp, and I was going to use that Hammond 229A24 transformer with a +/-15V regulated split rail power supply for the op-amps. The reverb unit would use a voltage divider and 5V regulator. I attached a schematic of a basic power supply I used in a different project as a starting point idea - I was basically just gonna slap that into this amp with a few tweaks.

[xtian]

Good call, my man! I used my back-up multimeter, and am reading 86mA for the total circuit, and 67mA for the BTDR only. Back to DEFCON 5. Stand down.

--

Replaced my batter in my Meterman 38XR, and it's showing 68mA overall, 39mA BTDR only.

[Theashe]

At least you didn't call your utility to have them check your transformer because mains was reading at 165V! That was an embarrassing call back.

I'm relieved it was just the multi meter. That Hammond transformer is for my attempt at this same project, only it's rated for 250mA. Would have had to buy another one. D:

[JazzGuitarGimp]

I'm glad you got it sorted out. I have to admit, you had me on pins and needles for a minute there. Got two BTDR-3H's in the mail a few days ago. Got all the misc parts from Mouser the day before that. Still waiting on the pc boards. I'm looking forward to working this reverb into my latest build.

[xtian]

Are you going to use a dedicated tranny for the PSU?

[JazzGuitarGimp]

For my initial testing and evaluation, I am going to use two bench supplies. If it stays in the amp, I have an unused 5V rectifier heater tap I will use with a voltage tripling rectifier circuit fed into a 7805 for the brick and a 7815 for the opamp.

[xtian]

Good good. I have an unused 5v winding also. You first.

[Phil_S]

I was going to say, jump over to my thread on the SE Plexi-verb. I'm using the 5V rectifier winding with a tripler. I've got roughly 7-14-21V depending on which diode. This works for both the 5V and 15V regulators very nicely. I'm seeing 4.98V at the 7805 and 15.0 or 15.1 at the 7815. There is a schematic posted there.

[martin manning]

http://ampgarage.com/forum/viewtopic.ph ... 949#356949 Probably want to use a low dropout regulator for the 5V when using a 5VAC filament winding for power.