Total Recall Gain 2 trim - volume always low at IC4, two separate builds

[lincolnic]

Hey gang, I'm wondering about the proper behavior of the Gain 2 trimmer in the Total Recall delay. I've built two of these so far -- one with four MN3008s, one with Xvive 3005s -- and in both of them, I can never get the volume at pin 1 of IC4 to match pin 7 of IC6. Everything in the biasing process goes well up to that point, but then with Gain 2 maxed out, the volume at pin 1 of IC4 is a good 6 dB or so lower than it is at pin 7 of IC6. This behavior happens in both of my builds.

The pedals still work otherwise, and generally sound pretty good (the 3008 one still has some clock noise at max delay time, which I need to figure out), but has anyone else run into this problem? Just how much of a problem is it? If it was just one build I'd think maybe I had a bad trimmer or something, but the odds that I picked two bad trimmers for the exact same spot on two different PCBs are pretty slim.

[Scruffie]

It's meant to drop some gain, there's a lot of gain in the delay path and we don't want that.

[lincolnic]

Is that so? The biasing instructions say "Connect your audio probe to pin1 of IC4_A. Adjust the GAIN2 trimmer until the output is the same or slightly above the volume input of pin7 on IC6." Are they incorrect, then? What's the proper way to set the Gain 2 trim?

[Scruffie]

That opamp can't give unity gain if it wanted to, 680k input resistor, 200k max feedback resistor.

Gain stage 1 should effectively be set up for unity gain between the BBDs, Gain 2 you want to match the dry and wet levels at the output basically, so either probing the blend pot or going between IC1_A Pin 1 and IC5_B Pin 7.

[lincolnic]

Thanks, Scruffie! I'll give that a shot. Hopefully it'll clear up the slight noise I'm getting at higher delay times.

[joesatch]

I'm having a similar issue. I used NOS MN3005's and it sounds fabulous. The thing is working as it should after i calibrated by ear but going by the Doc and using a probe doesn't get me the results when i probe pin7 or pin4 of IC4_B. Pin 4 is very faint and pin 7 gives me nothing. Could someone verify the instructions in the Doc about calibration?

[Scruffie]

Well IC4 pin 4 is supply voltage so i'd start by making sure you're checking the right pins.

[joesatch]

My bad yes, Pin 7 on IC4 gives me a very faint signal adjusting Gain1.

[lincolnic]

Outside of the instructions for Gain 2, the build doc is otherwise correct. With the Gain 1 trim, you're checking pin 7 of IC4_B against pin 7 of IC6.

[lincolnic]

Just wanted to pop back in and report that I redid the Gain 2 trim as per Scruffie's instructions, and the pedal is much happier now. I couldn't ever get the wet and dry signals to the same level (even with the Gain 2 trim all the way down, there's a 3 or 4 dB difference), but I think the circuit is sounding pretty nice.

Also, I want to confirm: is it normal to be hearing a little extra white noise around the repeats when the delay time is maxed out? I have a feeling it is, because analog, but I've never played an original DMM so I wanted to check.

[Scruffie]

My mistake, gain trim 2 should be adjusted so that pin 7 & 14/15 of the NE570 are equal level and then if that can't be achieved go back and tweak gain 1.

Yup, noise at full delay time is perfectly normal.

[lincolnic]

Thanks for the heads up! I'll go back and reassess that over the weekend, I'll probably be working all day tomorrow.

[daleykd]

@lincolnic, keep us posted.  I just finished my TR build last night, and had the exact same issue as you did at the beginning of the thread.  Even with GAIN2 at max (or min), the volume of IC4 pin 7 was never the same volume (as stated in the doc).

I also noticed another thread (http://www.madbeanpedals.com/forum/index.php?topic=21507.0) that talks about a bit of white noise (hiss) on the repeats.  This happens to me, even if the delay isn't MAXED.  If it's normal, I'm fine with that.  Some people say it's because I'm using the XVIVE instead of NOS.

[lincolnic]

I didn't have a chance to look at it over the weekend (maybe tomorrow?), but as stated elsewhere in the thread, the build doc instructions are wrong about how to set Gain 2. As per Scruffie's last post,

...gain trim 2 should be adjusted so that pin 7 & 14/15 of the NE570 are equal level and then if that can't be achieved go back and tweak gain 1.

Yup, noise at full delay time is perfectly normal.

I sent Brian a PM about the build doc, hopefully it'll be updated soon.

You should go back and revisit your biasing/gain trims and then see if you're still getting noise on all the repeats, or just at max delay time. I know my repeats cleaned up a lot once I had things more or less as they should be. I originally had Gain 2 maxed out, and it made things really noisy. I'm using Xvive 3005s as well, no complaints otherwise.

[lincolnic]

Forgot to post in here for the past couple of days, but it turns out that after I rebalanced Gain 2 to Scruffie's original suggestion (IC1/IC5), that was enough to make it right -- I went back and checked the levels at pin 7 and pin 14 of the 570, and they were exactly identical. So I hereby declare my 3005 build finished and sounding great!