BTDR-1H Rev module factory implemented in combo amplifier. Modded...

[HF1600ie]

Hi,
My amp (Hayden HGT-A20) brought a cheap reverb module fitted from factory, but I noticed that in the schematic and the PCB board it had written the BTDR-1 module with the correct size square for it! So, I checked and decided to buy one BTDR-1 (long decay version) as it seemed to be 100% plug and play !! And it was...

I tried to add the "Dwell" effect to it to extend the delay which doesn´t seem too long as it is, with a simple 47k pot wired as a variable resistor (1st and middle lug only) in the GREEN path of the modded file - from two different points before and after the module - but it doesn´t work. I am having either no extension at all or having sudden unpleasant oscillation / runaway.

How can I achieve this this? I´d like to set a "fixed" value and control the rest with the external "Reverb" control pot only to make it simple and mantain the external look of the amp.

Right now I am only able to get a not so loud kind of medium decay with the amp´s "reverb" knob set at middle or, if I max the "reverb" pot, I have the type of Surf echo I am not able to get a lush, spacey loooong decay. While I have decreased R33 (which I think is a mixer resistor), the reverb turns into a slap-back-surf type of reverb, with audible pre-delay, which is odd. I though if I decreased the value, more "clean/dry" signal would bypass the reverb module and move on, but it seems to work the opposite way. I also don´t understand if this resistor works as a feedback resistor (??).

I also have performed the modifications in the below file. But I wonder how to add a correct and working "dwell" control. I hear reverbs with long decays working with this BTDR-1 brick...but it´s not my case. I believe the base is in there, I just don´t know how to.

Some circuit seems to have buffered "dwell". Mine doesn´t.

Original Reverb circuit layout :



Currently modded version of the Reverb circuit with two possible Dwell configurations untested : Red or Green. Is the Red one, the correct one? Green is not working properly. It oscillates badly.



Thanks a lot for the help !

[ElectricDruid]

The red one is where I would start.

The trouble with the green one is you turn the feedback level up as you turn up the reverb level. So by the time you're getting much reverb signal, you also get runaway feedback and oscillations.

The C13/4n7 cap on the output of the brick is important. It's there because the Benton brick runs on a 5V single supply, and has a 2.5V bias point. The rest of the amp probably uses a +/-15V dual supply (at a guess). You need to keep those separate or you're also recirculating 2.5V DC! That'll slam the output into the rails pretty quickly. Put the cap in.

[samhay]

The BTDR bricks have AC coupled input and outputs.

If the 47k in the red option is a pot wired as a level control then it should work ok. You don't need the cap on the brick side (right) of this pot and you can reduce the 33k resistor to 10k in order to get unity gain feedback at max. I would also take the output from the other BTDR output just for a bit of extra flavour (the 2 outputs are not equivalent).

[ElectricDruid]

The BTDR bricks have AC coupled input and outputs.

Ah, ok. I didn't know that. Forget what I said about the cap then. That makes life easier.

[HF1600ie]

Hello,
Thanks for replying the topic! 

Yes, I was think about that. The "green" shortcut would increase the feedback signal when I turned up the reverb level. We want to separate those two, so the "red" should be the way to go, because it comes first.

My BTDR-1H (long), only has one output leg, I think. BTDR-2 has two outputs.
OK to a 10K fixed feedback resistor. I was being conservative with the 33K because of runaways...
I have no idea what the 100nF caps on the Dwell path do. Cutting high-frequencies? If I understood correctly, I can remove the 1st 100nF cap right after the BTDR-1 brick output (before the 47K pot)?


Some questions :

1 - I think the 47K DWELL pot can be wired with the 3rd leg to ground - as a level pot. Or, alternatively, with the 3rd leg empty, only as a variable resistor. Is the difference between these two options, the fact that if it is wired to ground, when it is at zero, I totally shut down the patch of the dwell and if I wire it as a variable resistor, when I turn it to zero, the other components are still active on the path ?

2 - What is the best way to control the tone (brightness) of the Reverb? Via the first cap C12 - 22nF ? I´m actually going to swap C12 with R18, if that makes sense. In all schematics, the capacitor is the last before the IC, not the resistor. But perhaps it´s the very same thing. Is R21 with a cap to ground an alternative to shape the tone of the reverb?

3 - Is R23 the mixer resistor? I have decreased from 33K to 5K or so after some testing. Why is it that I am getting a lot of "Pre-delayed surf-type echo" from the 60´s with the lower resistor? I think a lower resistor will allow my dry (clean) signal to bypass the brick, so, I should be having less "wet" signal. I don´t know why it seems to be working the opposite way.

4 - I removed R17 as I didn´t see in most schematics. Don´t know what it does, but probably it´s a "peaking circuit" along with C12 to boost some frequencies up a certain point ?

5 - I am having a bit of "hiss" when I crank my Reverb pot. Any reason ? Perhaps I should put C13 back on ?

6 - Any other recommendations to maximise the usage of this Reverb ? I find it pretty amazing and a clever solution that the Hayden HGT amp has a "Reverb pedal" incorporated. I´d like to keep it simple, perhaps with trim pots for Dwell and Dry/wet , inside.

Cheers!

[samhay]

>My BTDR-1H (long), only has one output leg, I think. BTDR-2 has two outputs.
Correct - has been a while since I looked at this version of the chip.

>If I understood correctly, I can remove the 1st 100nF cap right after the BTDR-1 brick output (before the 47K pot)?
Correct. This cap forms a high pass filter with the 47k pot and AC couples the brick's output to the pot/ground. As there is already a built in cap on the output of the brick, it is mostly redundant.

1 - I think the 47K DWELL pot can be wired with...
I would wire it like a volume pot - lug 1 to ground, lug 2 towards IC2 and lug 3 connected to the BTDR output.

2 - What is the best way to control the tone (brightness) of the Reverb?
I would put a cap parallel to R19. It will form a low pass filter against the 10k.
C12 forms a shelving high pass filter. I would leave this alone unless you want to cut more bass?

3 - Is R23 the mixer resistor?
Yes, R23 mixes the dry signal into IC3. I wouldn't play with this as you will end up overdriving one of the later stages. If you want to change the max level of the wet signal, play with R22.

4 - I removed R17 as I didn´t see in most schematics.
It forms the shelving part of the high pass filter. Without it you have a 1-pole high pass filter with a corner frequency of 720 Hz working on the dry signal hitting the brick. I would put R17 back or increase the value of R17 to taste (until the reverb is dark enough again).

5 - I am having a bit of "hiss" when I crank my Reverb pot. Any reason ? Perhaps I should put C13 back on ?
Not sure why you removed C13, but it's forming another high pass filter, this time against R22. This won't make much difference to the hiss, which is likely to be high frequency. The cap I mentioned in 2 will help, but you will get some amount of hiss with the BTDR chips.

6 - Any other recommendations to maximise the usage of this Reverb ?
I would keep the reverb control as you won't always want reverb. You may find you only like one setting of the feedback (dwell) control, but worth temporarily putting a pot in so you can play with it.

[HF1600ie]

Hi there,
Thanks for replying.

OK, I will let you know how it goes. This weekend I will try to install the "dwell" mod which is the most important.

1 - You are probably right. To make it simplier which is what I want for now, perhaps there´s one Dwell setting that I will like and I´ll set and forget. If this is the case, can I wire a fixed resistor in the place of the 47K pot? Do I still need to mantain the  feedback resistor "limiter" after the pot ?
Or, maybe, can I use a simple 100nF capacitor and a simple fixed resistor in this path ? Will this work, or I have to have a pot, so I can send some signal so ground and control the feedback signal ?

2 - I already have R19 as a 22K + 100pF in parallel copied from another schematic for the same location.
By the way, I removed R17 and swapped C2 with R18. It R17 really needed and does the order of C2/R18 matter? I think their order doesn´t matter.
So, to make it brighter, the shelving filter R17 is the best location to do it, right ?

3 - I think that decreasing R23 from 33K to 5K is creating a big of "dwell" to my reverb. I can be wrong. But, it seems to me that it is not letting more dry signal to pass through, but instead, it is letting more wet signal to go back to the start and re-enter the reverb brick again. Does this make sense?
Increasing R22 will probably "control" this path and its positive and negative flow. If I increase R22 with the stock R23 at 33K I will shut down (more) the wet signal´s normal path after the brick and also to allow it to re-enter back into the brick´s input again via R23.
Does this make sense? Can R23 be seen as a "dwell" ?

4 - Is R17 really needed ? If I can control the tone to be darker or less dark in R19, maybe R17 is only needed if I want a reverb brighter than normality. A smaller R17 will make the reverb brighter?

5 - Yes, the hiss is HF kind. Only near the maximum reverb settings. The original 100K pot is log which I think is not appropriate. The Reverb only works from 50% of the travel to the maximum. The last 20% of the travel make the most difference. I will switch it to a linear 100K. The usual BTDR reverb schematics use a 25K. Not sure which will work best. I decided to remove C13 copying other reverb schematic based on the BTDR.  Should I put C13 back ?
OK for the amout of hiss. Maybe I can tame it a bit by fine tuning the cap in parallel with R19. Currently R19 is 22K + 100pF. should I increase or decrease the cap for a darker tone ? I think to make it darker I should put a 1nF in parallel with the 22K in R19, for instance.

6 - Yes, I´d like to let is simply controllable via the external Reverb level pot. Just unsure how to dial a fixed dwell setting by using a simple fixed resistor (along with the 100nF capacitor)... if this is possible somehow.

Cheers

[samhay]

>Or, maybe, can I use a simple 100nF capacitor and a simple fixed resistor in this path ? Will this work, or I have to have a pot, so I can send some signal so ground and control the feedback signal ?

That will work. No needed to shunt signal to ground in this instance, but it might be worthwhile installing a trimmer as a variable resistor so you can tweak to taste.

Sorry, a bit lost with the rest. Any change you could draw out the schematic with the changes you have made?

[HF1600ie]

Hi Sam,
Thanks for the input.
I am using a 10K resistor in series with a 100nF in the Feedback loop. It seems to extend the decay, even though I feel no oscillation whatsoever, so I don´t know if it´s working properly.
I remember to max the 47K pot when I had it installed and it didn´t oscillate. Perhaps I am not placing the Dwell loop at the right place, am I ?

It is curious, before everything, if I maxed the Reverb level while decreasing R23 from 33K to about 10K, I would get a "pre-delayed" slapback which is interesting for surf music. But, right now, with the "dwell" loop, I had to put R23 back to 33K and set R21 as 47K. Otherwise, the "pre-delayed" echo change to very strong and unpleasant.
With R21 = 47K, the pre-delayed slap is softned and I only hear the decay which seems a bit prolonged with the dwell loop.

What´s interesting is that in the amplifier PCB board, R21 has written "33K", in the schematic R21 is 100ohm BUT the real resistor installed from factory was 1K !!! What a mess :-)

I didn´t touch R22 and I have R23 set back to 33K (the factory spec). I believe I will set a trim pot in R22 or R23 for fine tuning.

But, everything seems to be balanced right now. Tonewise it is also good. It seems less harsh now without the R17 making the shelving filter at the beguinning. Perhaps it was boosting some high-mids...

Still wondering about the feedback loop. Something´s not right... 

[samhay]

That all looks ok.
IC2 is set up as an inverting mixer, so the feedback loop design is perfectly valid. The gain of the wet signal (feedback) should be 1.0, but either the brick has gain < 1 or you have enough variance in resistor values that it isn't oscillating.

[HF1600ie]

Then, that means I am not getting back the longest decay possible.
Right now, it seems to me I can hear the delay for about 3 seconds. But.. hey! This is what it was supposed to do without the "Dwell" control, right ? My brick is the "long" version. Unless there´s more, but not audible enough to be perceived.

If I could hear a pre-oscillation behaviour I knew that a slightly bigger feedback resistor would be all i needed...

[samhay]

What happens if you make the feedback resistor less than 10k?

[HF1600ie]

I remember to turn the 47K pot while it was installed from 0 to max and while at zero the delay was short and after turning the pot a little bit it got longer. But not to a point where I could control the decay time with precision and have a runaway at the maximum setting. It was more like an ON/OFF operation. Is this normal?

On the other hand, I tried the dwell in different locations with that 47k pot wire as a variable resistor and had runaway. I wonder if I should pick different locations.

[samhay]

There isn't a great alternative place to take the feedback from as you mix dry signal back in at the next mixter/active stage.

The resitor between the 47k pot and (-) input of IC2 will determine the max gain/runaway behaviour.
If you make the 10k resistor slightly smaller, you should get oscillation. You could then put a pot back, either as a variable resistor or as the 47k pot to ground, to make this more controllable.

[HF1600ie]

I remember to place only a 47K pot at the "green line" - see pic earlier this thread and while rotating it close to the maximum, it would oscillate.

Doing the same procedure where I have it right now, doesn´t seem to be so agressive.

Perhaps R22 or R23 should be tweaked to extend the decay - at least the perception of it?

I have plenty reverb "volume" right now and the pre-delay "echo" seems to be well controled with R21. Just wanted a tad more extended "decay". Not a lot more really. Tone seems to be right as well.

Kind of stuck at the moment   

[ElectricDruid]

This can't really be super-complicated. You've got a pre-delay mixer to mix signals going in, and you've got a post-delay mixer to mix signals coming out. These circuits behave as we expect them to, for the most part. Which helps

If the amount of signal going into the pre-delay mixer from the delay output is sufficient, you'll get feedback. If the delay itself has a gain <1, then the pre-delay mixer needs to compensate (e.g.  Feedback resistor 10K needs to be *less* than 10K). Personally, I'd be inclined to assume a level less than 1, use a resistor of, say, 6K8 or 8K2, and then add a pot (signal to the top, wiper to the feedback resistor, bottom to ground) to control the feedback level so that I could juggle it and get it just right. The only trouble with this is that a input impedance of else than 10K on the pre-delay mixer will mess with the pot to a significant degree, but you'll probably get away with it.

On the output mixer, you've got a pot(VR12) that mixes in the delay's output via resistor R22/68K. That's all good and shouldn't need any alterations unless you're not getting enough delay in your signal (or want the delay louder than the input) in which case reduce R22 - try 56K or 47K for starters, 33K or 22K if that's not enough.

HTH,
Tom

[HF1600ie]

Hi,
Is the pre-delay mixer R18 ? Is the post-delay mixer R21 ? What are R22 and R23 then ?
R23 seems to allow signal bypass the brick, but also let the "post-reverb pot" signal re-enter the pre-brick signal, depending on how we look at it. R22 seems to back him up. A bit confused 

I had more or less made a copy from other BTDR pedals. But, just wondering how far could the feedback loop extend the decay but being under control. It has to be useful for a normal application. Also, the mixer resistors above could play a very important part.

My currently applied values, are :

R18 = 10K
R19 = 10K
R21 = 47K
R22 = 10K.
R23 =33K
R25 = 10K
Fixed feedback resistor (no pot) = 10K in series with 100nF

[ElectricDruid]

The pre-delay mixer is IC2/uA741. It's set up as an inverting op-amp mixer circuit. R19 is the op-amp feedback resistor, and you've got R18 and your new 10K resistor as the input resistors for the input signal and the delay feedback signal.

The post-delay mixer is the same arrangement, but around IC3/uA741. This time, R25 is the op-amp feedback resistor, and R23and R22 are the input resistors for the dry and wet paths respectively.

You're dead right that the mixer resistors will play a very important part, especially if you're trying to balance it pretty finely so that you get lots of feedback but without it running away.
The 10K delay feedback resistor is the critical one for that. The post-delay mixer resistors only set the balance between the wet and dry signals, but won't change the *content* of that wet signal (e.g. tweak 'em as much as you like and you won't get any more delay - you'll just get the same delay louder or quieter).

HTH,
Tom

[HF1600ie]

Hi,
Actually I am not using UA741 anymore. I swapped all those 3 opamps in the circuit for TL071 and TL081.

Well, I had a look at it this weekend and ended up increasing R22 from 10K to 22K as a first test. I wanted to "hold on" a little bit more the wet signal coming from the brick into the exit opamp, so that the dry signal could "stand" out a little bit more via R23.

I hope my interpretation is correct. If it is, I will probably make it 50/50, which means increasing R22 to 33K, the same value as R23. It would be so cool If I could use one single trim pot which would control R22 and R23 alone !! But if this is not possible, I can always place it in the R22 place.

I wonder If I can look at R23 with the signal going in both directions (going back into first opamp but also as brick bypass)  This is because I remember long ago decreasing it to 5K or so and apparently my wet signal got more presence (more pre-delay, more effect, etc). Same with the opposite.

R21 seems fine at 47K. It is enough to tame down that first "slapped pre-delayed echo" which, when uncontrolled can become more audible that the played note in the guitar itself ! I actually need that pre-delay a bit to account for the global reverb effect - but there has to be a balance...otherwise it makes the final sound confusing.
The guitar sound has to stand out.

I will try to decrease the feedback resistor to 7.5K and see if there´s any improvement in the decay extension. Maybe not... 

Some clips may come very soon. 

[ElectricDruid]

Actually I am not using UA741 anymore. I swapped all those 3 opamps in the circuit for TL071 and TL081.

Fair enough. Probably a good choice. uA741 isn't the greatest op-amp in the world any longer, not in a long while. TL071 and TL081 should be quieter.

Well, I had a look at it this weekend and ended up increasing R22 from 10K to 22K as a first test. I wanted to "hold on" a little bit more the wet signal coming from the brick into the exit opamp, so that the dry signal could "stand" out a little bit more via R23.

Sounds reasonable to me.

I hope my interpretation is correct. If it is, I will probably make it 50/50, which means increasing R22 to 33K, the same value as R23. It would be so cool If I could use one single trim pot which would control R22 and R23 alone !! But if this is not possible, I can always place it in the R22 place.

You *could*. It's a bit unusual, but if you connect a trimmer with the wiper to the op-amp input, and apply the signals to the two ends, you can vary both input resistors in an either/or manner. With 68K feedback resistor, a 100K trimmer might be a good choice (very literally your "50/50"!!). Adding a small fixed resistor on both the ends of the trimmer would stop things going strange when you hit "zero" at one end or the other. Maybe 4K7?

I wonder If I can look at R23 with the signal going in both directions (going back into first opamp but also as brick bypass)  This is because I remember long ago decreasing it to 5K or so and apparently my wet signal got more presence (more pre-delay, more effect, etc). Same with the opposite.

No, it only works in one direction, from the input to the output mixer. There's no signal going back from the delay output to the first op-amp.

The reason for this is that the input of an op-amp mixer is a "virtual ground node", which means that there is a voltage of zero there. The only way this is possible is of the currents through R22 and R23 are matched and cancelled by an equal-but-inverted current through R25. And the only way that can happen is if the op-amp is outputting a voltage which is the matches the mixed-gain inputs from R22 and R23, which of course is exactly what it does do.

R21 seems fine at 47K. It is enough to tame down that first "slapped pre-delayed echo" which, when uncontrolled can become more audible that the played note in the guitar itself ! I actually need that pre-delay a bit to account for the global reverb effect - but there has to be a balance...otherwise it makes the final sound confusing.
The guitar sound has to stand out.

TBH, I'd remove R21 altogether and put a jumper in. All it does is limit the top end of the pot. At the moment with 47K+100K, you've essentially got a pot that only goes up 2/3rds of the way. You can compensate that later by changing R22, of course, but why bother and why make things more complicated?

I will try to decrease the feedback resistor to 7.5K and see if there´s any improvement in the decay extension. Maybe not... 

If you're not getting runaway feedback with 7K5/22K on that pre-delay mixer, I'd start to think there's something wrong. Have you got a pot controlling that level too, or not?

Some clips may come very soon. 

We look forward to it!

T.