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2021-08-04 07:08
@bjm
Thanks for that...I really appreciate it. I've even tried changing a couple resistors that I thought might have been the issue, but no luck so far. Let me know if you have any ideas.
Cheers,
Mike
2021-08-11 04:14
Just checking back. I've double checked the pot values, and replaced one capacitor I thought might have been the issue. No luck so far. Any thoughts on what might cause the volume to drop after 3 oc on Dwell 1?
Thanks, Mike
2021-08-19 15:50
Hi there,
OK so at deeper investigation:
Firstly slight volume drp when increasing Dwell 1 wasn’t considered a problem because it also gives more reverb and volume goes slightly up as reverb goes up.
Now anyway having a look at how to combat this :
A JFET source follower is chosen for input buffer because it has lower noise than an OP amp buffer and input impedance independent on load at output but it has a finite outputimpedance and voltage gain is less than 1 times and there is a trimmer gain for output stage to give gain through circuit at unity
So looking at T1 it can drive about 2K Ohms before source impdedance introduces a loss.
We can minimize this loss by firstly increasing current in the J Fet T1 by lowering R4 to 2K2. We can further reduce load by scaling up R7 and R 8 by a factor 10 so that R7 becomes 100K and R8 becomes 47K
Looking at the inputimpedance of T3 it consists of R7 //R8// IZ base meaning impedance at base in parallell with R7 and R8 ( this is because for AC signal B+ and ground are the same point: both are ground to AC signals)
Izbase is roughly hfe xRE+ hie. RE is the total emitter load approximately 1K8 and typical hfe for a BC109C is 400 and Rie from data sheet 4-6K Ohms. It follows that stability is good enough with R7=100K and R8=47K and new iZ of T4 becomes about 30K instead of about 3K3.
Then C5 needs to be scaled down by a factor 10 to keep the same corner frequency at about 110Hz
It can be good to remember that a reverb tank has a small bandwitdth from say 300Hz to 3Khz ( actually typically lower). For this reason it is no real benefit to drive reverb tank with a lot of bass as at worst that will saturate the transformers
I would suggest trying 15nF to 22nF
For T2 I could suggest using 470nF for C3 and also to install a resistor between Dwell 1 and C3. Resistor value could be 1K5
T2 is a smallpower transistor capable of driving high impedance tanks. Input Z is in this circuit slightly lower than hie of T2 so roughly about a few KOhms
Of course T1 has had to drive about 1K5 and that would make a loss of volume
With the changes suggested above load at source of T1 goes up to well above what T1 can drive and further increasing current through T1 will increase drive capability
Getting really into the neat parts T3 is a phase splitter and signal peak is slightly higher at collector than at emitter. The two phases can be better matched if R9 is made 2K0 and R10 made 2K2
Have great fun
BJ
2021-09-18 11:24
Hej Björn (and others on the forum too)!
I had an attempt at building the Universal Reverb kit a while back, but have issues with the 8 ohm output that I'd like to sort out. I have the v 1.3 PCB running into a brand new TAD 9AB3C1B reverb tank (has the 8 ohm input).
Most parts of the circuit seem to be working. The input and output stages seem to be passing the dry signal through just fine. C- out is driving the tank (not very optimally of course) to a degree, and this is picked up by the recovery stage. Shaking the tank also produces a very strong signal at the recovery.
It's the 8ohm/transformer- output that is not working at all. If I don't have the tank connected to the 8ohm output and TR1 is not fully clockwise, there is a fairly strong signal at the 8ohm pin. However, as soon as I connect the tank the signal goes completely silent at the 8ohm pin. I'm also not able to pick up any reverb from the tank, either using the recovery stage or with an external mixer. This is independent of the Dwell 1 setting.
One weird effect I'm getting is that that whenever TR1 is not fully clockwise, I get a little bit of treble-heavy dry signal at the reverb return. This happens whether the tank is connected or not. However, if I connect the tank to the 8ohm output (leaving the return unconnected), this trebly signal becomes louder. This trebly signal at the return also gets louder as Dwell 1 is increased.
Any ideas on where to start troubleshooting? Any voltages etc. that should be checked? I don't have a particularly deep understanding of electronics, but I but have access to a DVM and using an audio probe to trace the signal. This seems to be a really cool device and would like to hear it working properly.
I would really appreciate any ideas 🙂
Best,
Oliver
2021-09-26 14:09
Hi Bjorn,
Thanks for your previous instructions. I made the modifications you recommended: R4 to 2k2, R7 to 110k, R8 to 47k, C5 to 15n, C3 to 470nF. The one instruction I need clarification on is where to put the 1K5 resist0r between Dwell 1 and C3. Should it go to Dwell 1 a or Dwell 1 b? From the + of C3?
This fixed the volume drop that was happening previously, but now the reverb tank isn't getting signal from the guitar. I can hear the reverb tank when I touch the springs, so the return is working. The guitar signal sounds normal (no volume loss), but there is no reverb on it.
To be doubly sure, I purchased the same kit again and rebuilt it meticulously, triple checking all instructions, and scrutinising all joints along the way. The new kit I built ended up having the same issue as the first--volume loss when Dwell 1 is turned up past 3 o'clock.
Sorry to trouble you with all this, but it's definitely frustrating for me.
- Firstly, can you please tell me where to correctly connect the 1k5 resistor between Dwell 1 and C3?
- Secondly, any thoughts on why signal isn't getting to the reverb tank?
- Thirdly, with the new build, I don't understand how I could have gotten the same result of the volume loss on 2 separate builds. Any ideas? If that's not normal, is there a way I can moody sounds build me one that functions correctly? I'd love to have a fully functioning pedal. 🙂
Thanks,
Mike
2021-09-26 17:48
Hi there gents
I’ll combine answers below
Hej Björn (and others on the forum too)!
I had an attempt at building the Universal Reverb kit a while back, but have issues with the 8 ohm output that I'd like to sort out. I have the v 1.3 PCB running into a brand new TAD 9AB3C1B reverb tank (has the 8 ohm input).
Most parts of the circuit seem to be working. The input and output stages seem to be passing the dry signal through just fine. C- out is driving the tank (not very optimally of course) to a degree, and this is picked up by the recovery stage. Shaking the tank also produces a very strong signal at the recovery.
It's the 8ohm/transformer- output that is not working at all. If I don't have the tank connected to the 8ohm output and TR1 is not fully clockwise, there is a fairly strong signal at the 8ohm pin. However, as soon as I connect the tank the signal goes completely silent at the 8ohm pin. I'm also not able to pick up any reverb from the tank, either using the recovery stage or with an external mixer. This is independent of the Dwell 1 setting.
One weird effect I'm getting is that that whenever TR1 is not fully clockwise, I get a little bit of treble-heavy dry signal at the reverb return. This happens whether the tank is connected or not. However, if I connect the tank to the 8ohm output (leaving the return unconnected), this trebly signal becomes louder. This trebly signal at the return also gets louder as Dwell 1 is increased.
Any ideas on where to start troubleshooting? Any voltages etc. that should be checked? I don't have a particularly deep understanding of electronics, but I but have access to a DVM and using an audio probe to trace the signal. This seems to be a really cool device and would like to hear it working properly.
I would really appreciate any ideas 🙂
Best,
Oliver
Hi There Oliver,
Let’s see how I can help you
Firstly the TR1 is the bias voltage to the push pull pair that drives the transformer and this should be adjusted from minimum ( when viper of trimmer is connected tto ground) until transistors start to conduct resting current.
Having this set just where transistors open gives the widest signal further increasing TR1 will add to heat and saturating the transformer: therfore this trimmer has a limiting resistor.
So this a circuit that can be tested part by part because it can be broken down in outputs and so if signal is sent to input
Input stage can be monitored at Dwell 1
C drive output can be monitered at C out
Transformer output can be monitored at the 8 Ohm output
FInally signal should appear at output
Now Recovery stage can be tested seperately by putting signal into Reverb Recovery ( remember output from a reverb tank is very small so recovery amplifier has ample gain and not of an input will drive this input so it can be monitored at output of circuit
OK a rare fault in tanks is that input is shorted, producing exactly what you describe. This is a bit hard to measure at the input of an 8 Ohm tank, but it can sometimes be seen and I had this problme on a Bang Olofsen tank and there was a little flange that would hold the input wires in place but it at the same time shorted the two wires
This was an old tank from times when home music equipment sometimes had reverb add on.
I suggest first check that the tank works and that it does not have a shorted input
If shaking the tank prodces a loud sound then recovery stage is working
Yes there can be some leakage because the mixing is done passively just like in old equipment and that is done this way to attribute to the sound-it will be a part of sound once the whole circuit works. Because this is a discrete circuit the Power Supply Rejection Ratio is pretty low-unlike circuits with OP amps so there can be a bit of leakage trhrough the B-tree just like in tube circuits and other discrete trsnsitor circuits.
Hi Bjorn,
Thanks for your previous instructions. I made the modifications you recommended: R4 to 2k2, R7 to 110k, R8 to 47k, C5 to 15n, C3 to 470nF. The one instruction I need clarification on is where to put the 1K5 resist0r between Dwell 1 and C3. Should it go to Dwell 1 a or Dwell 1 b? From the + of C3?
This fixed the volume drop that was happening previously, but now the reverb tank isn't getting signal from the guitar. I can hear the reverb tank when I touch the springs, so the return is working. The guitar signal sounds normal (no volume loss), but there is no reverb on it.
To be doubly sure, I purchased the same kit again and rebuilt it meticulously, triple checking all instructions, and scrutinising all joints along the way. The new kit I built ended up having the same issue as the first--volume loss when Dwell 1 is turned up past 3 o'clock.
Sorry to trouble you with all this, but it's definitely frustrating for me.
- Firstly, can you please tell me where to correctly connect the 1k5 resistor between Dwell 1 and C3?
- Secondly, any thoughts on why signal isn't getting to the reverb tank?
- Thirdly, with the new build, I don't understand how I could have gotten the same result of the volume loss on 2 separate builds. Any ideas? If that's not normal, is there a way I can moody sounds build me one that functions correctly? I'd love to have a fully functioning pedal. 🙂
Thanks,
Mike
Hi there Mike,
I see sometimes problems do not have to be complicated to be very frustrating
This resistor is works as minimum resistance on the Dwell control and it does not matter if it comes before or after the Dwell pot that is wired like a reostat, meaning just variable resistance
I think there might be a problem at the Dwell pot: you can wire it so that the 1K5 resistor goes from DW1 to terminal 2 on Dwell pot ( that is middle terminal and also the viper inside) and so this resistor replaces the wire from Dw 1 to dwell pot and then run a wire from treminal 3 ( this is the terminal that viper connects to when potentiometer is turned fully clockwise) .
The reason I think it might be at Dwell pot is because how you describe it and that you can hear the springs suggests that recovery and mixer is working just fine and that guitar signal comes through but no reverb suggest that problem is at input of reverb
OK have you on the second build made that according to instructions I left and that you used on the first build in other words are they exactly alike?
If they are that would not be normal at all-they should be behave alike
At your service
BJ
2021-10-07 04:48
Hey Bjorn,
Thanks for your response. To clarify, you said 'the 1K5 resistor goes from DW1 to terminal 2 on Dwell pot ( that is middle terminal and also the viper inside) and so this resistor replaces the wire from Dw 1 to dwell pot and then run a wire from treminal 3 ( this is the terminal that viper connects to when potentiometer is turned fully clockwise)'
Where does the wire from terminal 3 connect to?
Thanks,
Mike
2021-10-09 03:36
Hi there
I'm working on the Universal Reverb kit and was planning on mounting it together with the reverb tank in larger rack style enclosure. I was thinking it would save a bit of space inside if I wired RCA cables directly to the pcb, and was hoping someone on here had some insight about how I'd do that (or if that's even possible). When I opened up the cables, I realized I know basically nothing about how RCA works...from what I gather the inner cable is the signal, and the surrounding wire is ground. This may be a stupid question, but could I just solder the inner cables to "c" and "return" on the pcb and gather the outer threads and solder to one of the extra ground holes?
Thanks!
2021-10-11 16:47
@mbeach I'm interested in hearing your thoughts on these mods once you get it up and running. I have everything built and am having similar issues; volume drop with Dwell 1 maxed, etc.
2021-10-13 22:33
@cmchaney06 I'll keep you in the loop with the mods. When I did the first build, I thought that volume drop was an error on my part. I think it was part of the original design....it makes sense, as the added reverb helps balance out the volume. I've got parts on the way to build another one though, so I'll let you know how it turns out.
Topic starter
2021-10-20 07:55
Hello, we have found a simple mod to improve the sound when the dwelll1 knob is turned to max. The resistor in the first buffer stage has too high value (10k) in the versions before v.133. When dwell 1 is turned to max, the following stages loads down the first stage too much and that is the reason for the sound quality going down. You can change the 10k at the source of T1 to a 2k2 and it will get a better. There are other modifications that can be done if you are having problems with a volume drop, but I recommend the 10k->2k2 to start with. Here is a list of mods to versions before v1.33
Topic starter
2021-10-20 08:06
@busterjbrown82 Hi yes that sounds like a way to go. I would say you can skip the jacks a wire the tank directly to the board. There are extra holes for ground on the top left corner of the board the you can use for the tank ground.
2021-12-11 21:31
I am just finishing building my BJF Reverb kit, and I was wondering if there’s an “official” method of setting bias via TR1 and TR2. I have an oscilloscope if that helps. Thanks!
2021-12-12 20:29
I have finished building the pedal now, but I'm running into an issue. The recovery amp seems to be working perfectly (I can clearly hear the "splash" when I flick the springs, and the "Dwell 2" knob functions correctly), but I don't hear much reverb on my guitar. I think there must be something going on with the send amp. I have used an oscilloscope to look at the output of the transformer out (I am driving a 10ohm tank), and I see a small signal. I also see it at the return. It seems like there's not enough level driving the tank though? I tried adjusting TR1, but it just increases distortion, without increasing the level much. Any ideas? Thank you!
PeterB
2021-12-13 08:33
Hi Peter,
The transformer output is made to drive 8 Ohm tanks so yes it might be too low for 100 Ohm tanks.
Ideally if you can find you'd need something like Hammond 149S audio transformer
But you can also try capacitor output but then values and bias may need to be altered
At your service
BJ
