Hello,
Was hoping I might be able to pick some brains for answer to the cause of squealing with the ruby. If I have the volume knob turned up at all past the first little bit I get an absolutely awful squeal from the speaker. I used my audio probe and the signal traces out fine until lug 2 of the volume pot at which point the squeal begins. If I turn the volume knob down to the point I don't have the squeal I can hear guitar through my speaker although it sounds somewhat distorted. If I have the volume knob turned so there is no squealing and audio probe it I get a fairly clean signal all the way to the speaker terminal. This was not really a planned project so I got the transistor and ic at my local shop. For the mpf102 I am using an HEP802 which I was told and haven't been able to back up is a nte312 equivalent which I can see is an equivalent for the MPF102. In digging around I found this thread https://www.diystompboxes.com/smfforum/index.php?topic=97634.0 where he was having a similar problem (messing with the gain pot can cause squealing on mine when I'm audio probing it). Also I am using a lm386-n1 also what I got from local place last one they had from Tandy corp lol. Both pots are as listed in schematic 10kb for volume 1kb for gain. I tried a 1k for volume due to reading online that using a 25k for volume could lead to squealing no change. Also I did my own breadboard layout, could get it to work at all using beavis audio layout, noticed some errors in that layout vs the schematic. Output cap size, volume 2 pot going to pin 1 of ic. Gain pot two should be going to ic pin 1 with just 3 going to 8. Here are my voltages:
D- 9.27
S- 1.88
G- 0
1- volume knob maxed (full squeal) 1.13 squeal stop 1.27, all the way down 1.31
2- 0
3- 0
4- 0
5-1.32
6-4.69
7-9.27
4.47
And some pictures:
(https://i.postimg.cc/rR843vdP/3-EECDC8-D-E17-B-4-EEB-AA6-D-81429-F9-BDB6-C.jpg) (https://postimg.cc/rR843vdP)
(https://i.postimg.cc/mPcHv3DB/5-D3-EF443-01-E9-4931-912-C-815016-FBEBE2.jpg) (https://postimg.cc/mPcHv3DB)
(https://i.postimg.cc/JGzJqCCy/7760-A3-A6-3-A10-4148-9-A19-122-C825-DFAAA.png) (https://postimg.cc/JGzJqCCy)
(https://i.postimg.cc/mcCFDbCn/912-DB403-F8-B9-41-A2-A027-231-AA77-E4464.jpg) (https://postimg.cc/mcCFDbCn)
(https://i.postimg.cc/WdQkTDFr/94-EF381-E-F888-4629-9695-9-C48-A9-C30829.jpg) (https://postimg.cc/WdQkTDFr)
(https://i.postimg.cc/zV2h22qx/C3-F744-EF-4-CB1-4-E75-9169-0-C46-D12-B252-C.png) (https://postimg.cc/zV2h22qx)
(https://i.postimg.cc/6yP7f7hS/B8-B2-DD29-8-A09-4396-875-F-1-B23-F274779-E.jpg) (https://postimg.cc/6yP7f7hS)
I should start from Q1 Source voltage by making R2 4k7 (or better 5k6).. :icon_wink:
(not relevant to squeal at a first glance but let's try to fix LM386 input signal amplitude and go on..)
Alright we now have
9.27
1.95
0
Not good..
We aim to over 3V.. :icon_wink:
Hmm. Maybe I should put this on hold until I do another parts order and can get the j113
Would connecting R1 to some kind of Vref help ?
Try these anti squeal tips
Put a cap in parallel to R1
Put a 100uF and 0.1uF across your power rails
Reduce R1 to 1M
Reduce length of all cables / use shielded cables
Make sure input is far from output
Thank you. I'll try reducing the resistor on r1 I already have the 100uf cap running from + to ground not in schem. With no connection to ground I'm only getting 2.32 there.
Isn't pin 6 of LM386 going to +9, hence you should measure +9 at that point ?
Oops 5=8, 6=7 etc
Tried a 2nd one of those transistors. I bought 2. It dropped to 1.27...
Looks to me like your volume control wires run right over the '386 OUTPUT, then back to the '386 input.
Just like wells and outhouses, layout matters.
Re FET voltage, please try connecting one end of R1 to a voltage divider with cap (same configuration like a vref for Opamp)
Quote from: PRR on July 10, 2022, 09:20:54 PM
Looks to me like your volume control wires run right over the '386 OUTPUT, then back to the '386 input.
Just like wells and outhouses, layout matters.
That's actually the gain control. But you are correct and the input wire is joining the party. I can shuffle that around to see if there's any improvement in the stuck pig effect I'm getting as well.
Quote from: Vivek on July 11, 2022, 01:36:48 AM
Re FET voltage, please try connecting one end of R1 to a voltage divider with cap (same configuration like a vref for Opamp)
I'll have to look that up. Still very green, but I'll check it out tomorrow. Just trying to make a little amp for my bench so I'm not lugging my other one back and forth.
Place the 100uf power supply cap as close to pin 6 as possible.
Quote from: Vivek on July 11, 2022, 01:36:48 AM
Re FET voltage, please try connecting one end of R1 to a voltage divider with cap (same configuration like a vref for Opamp)
I'm trying to wrap my head around exactly what you are suggesting here. Are you suggesting attaching r1 to something like this.
(https://i.postimg.cc/crnxWv68/66-ED2-B0-C-6-BE3-4543-B2-FB-73938-F831-C2-F.png) (https://postimg.cc/crnxWv68)
And then adding a capacitor to that in series prior to connecting r1 to it? And using r1 to bias to the desired place of at least 3v? Am I aiming for 3v or is there a window I am shooting for here. I'm assuming the purpose of the resistor to ground (r1's current function is to pull excess voltage from the source pin). So r1 would no longer be going to ground but the + voltage source from the vref? Which I can set to be whatever I want by changing the resistor values with that ohms law calculator? Pretty much a total beginner here I'm sure this is pretty basic stuff.
Or was it more something like this idea.
https://www.electronics-tutorials.ws/capacitor/capacitive-voltage-divider.html
Nevermind you mean the power cap between the positive and ground I think.
Quote from: Locrian99 on July 11, 2022, 07:52:11 PM
Nevermind you mean the power cap between the positive and ground I think.
Paul Marossy's LM386 battle,
https://www.diystompboxes.com/smfforum/index.php?topic=120343.0
Thank you I'm going to try moving that cap on the power rails right next to where pin 6 comes off. Funny though he mentioned the new chip behaving differently than an Old. Here's the package my chip came in. Nothing new about it.
(https://i.postimg.cc/4HYYZRx5/2-E92-B7-D4-B3-D6-4-C98-8-EAE-A9-E18-D15-AE4-E.jpg) (https://postimg.cc/4HYYZRx5)
(https://i.postimg.cc/KkvKqMXC/A59-BFC80-C587-4-EFA-A73-E-B19-A9-A16-ED89.jpg) (https://postimg.cc/KkvKqMXC)
I should have a few minutes here in a bit to dink around with it.
The squealing was fixed by changing layouts. I just went back to the spread out way on beavis audio and fixed the errors in it rather than doing a whole new way. No more squeal. Low volume and distortion though. I'll play with the voltage divider in a while I got it to raise the voltage but was getting no sound. Does the source pin have to have some ground connection?
> Does the source pin have to have some ground connection?
It does/should; through R2 3.9k.
(FWIW: the drawing had a "error", the internal source lead didn't go all the way to the chip. But that would not stop you from running the S pin to R2 to ground.)
(https://i.postimg.cc/87Sn3jMg/Ruby-Amp-Drain-42.gif) (https://postimg.cc/87Sn3jMg)
Hey PPR thank you for the layout suggestion you were spot on. So I've tried this a few different ways. 3.9k straight to ground as the layout/schematic read and I get sound it's very low and distorted (. I tried following the suggestion of a voltage divider witch cap and running that straight to the source strip, and upping the resistor to ground to 8.2k (this got me to 2.69v) No sound. Tried a higher value resistor without the voltage divider back to low volume distortion. Tried Antonius suggestion of 5.6k got me around 1.9. Left just the 8.2k in got 1.98 still low volume distortion. None of these seem to change the 1.3v at pin 1 of the lm386 though or the 4.59 at the output. Not sure if that matters. I'm suspecting this HEP802 is not a direct equivalent for the mpf102 at this point.
It's worth checking your JFET pinouts, see attached pic.
Based on your measurements I estimate the JFET has a VP of about 2.4V.
FWIW, the specs for either the MPF102 or HEP801 are all over the map.
Old school JFETs had very loose specs. Modern day versions tended to be
tighter than the datasheets in practice.
(You can see some MPF102 stats at, https://runoffgroove.com/fetzervalve.html, near bottom of page.
You can see the specs are much wider, https://www.alldatasheet.com/datasheet-pdf/pdf/53879/FAIRCHILD/MPF102.html)
I've got doubts that you can get this to bias without a divider on the gate like this:
(https://i.pinimg.com/736x/8c/3f/21/8c3f21c0945f2c8092a6c3f95fdf4714.jpg)
If you are going to do that you might as well leave the original 3.9k source resistor in.
You might need to lower the ground side gate resistor to tune the source voltage, maybe around 1M2.
Use 2M2 for the top resistor.
Here's all the info I have on the HEP801,
(https://i.postimg.cc/LYWn1SBY/Motorola-HEP-JFET-1973-Info.png) (https://postimg.cc/LYWn1SBY)
You can also use an ordinary transistor in a bootstrapped emitter follower circuit. Below is one with Zin of about 1.3 Megaohm:
(https://i.postimg.cc/QBq8H49W/Boot01.jpg) (https://postimg.cc/QBq8H49W)
Quote from: Clint Eastwood on July 12, 2022, 06:20:50 AM
You can also use an ordinary transistor in a bootstrapped emitter follower circuit. Below is one with Zin of about 1.3 Megaohm:
(https://i.postimg.cc/QBq8H49W/Boot01.jpg) (https://postimg.cc/QBq8H49W)
Rc could be omitted (in fact, it MUST be omitted 'cause if you try to bias Emitter near around 4.5V you'll result into a saturated BJT..) unless the "buffer" above is in fact part a CE amp folowed by a buffer.. :icon_wink:
(https://i.imgur.com/8zTt1Eo.jpg)
You are right Antonis. I did a quick redesign of the circuit, I think it is better now:
(https://i.postimg.cc/2VByYG5Y/highimp.jpg) (https://postimg.cc/2VByYG5Y)
Input impedance is about 1Megaohm, emitter voltage 3,3 volts.
It should be even better if you interchange R2 & R3 values and make R5 3k9 - 4k7 .. :icon_wink:
(the lower the Emitter resistor value the higher the working current hence the bigger the load drive capability - translated into "linearity"..)
P.S.
See Figure 1. configuration (B) here: https://www.diystompboxes.com/smfforum/index.php?topic=129250.0 (https://www.diystompboxes.com/smfforum/index.php?topic=129250.0)
Thank you for the link Antonis. I see you are an expert on the subject :icon_smile:
Quote from: Clint Eastwood on July 12, 2022, 10:46:10 AM
I see you are an expert on the subject :icon_smile:
.............................................................................
edit: Just realized how badly it sounds in English.. :icon_redface:
(sorry, no offence intention..)
Thank you guys for taking the time to throw out some ideas. From your schem rob I can see I was doing the voltage divider thing wrong last night I was running the voltage output from it straight to the source pin rather than through the gate. Probably explains why it killed my signal. I'll try messing around with that. Is 3v at the source pin the bias goal here. Obviously I can use my ears, but voltage readings just seem so much more tangible. The emitter follower circuit, is the transistor part crucial or should I just be able to throw something there. Like a 2n2222?
Quote from: Locrian99 on July 12, 2022, 04:51:42 PM
The emitter follower circuit, is the transistor part crucial or should I just be able to throw something there. Like a 2n2222?
Generally speaking, not so crucial..
But a high beta (h
FE) transistor could afford higher value bootstrapped bias resistor without too much voltage drop..
(Voltage divider Thevenin equivalent resistance could also be neglected so divider and Base voltages could be considered identical..)
QuoteThank you guys for taking the time to throw out some ideas. From your schem rob I can see I was doing the voltage divider thing wrong last night I was running the voltage output from it straight to the source pin rather than through the gate. Probably explains why it killed my signal. I'll try messing around with that. Is 3v at the source pin the bias goal here. Obviously I can use my ears, but voltage readings just seem so much more tangible. The emitter follower circuit, is the transistor part crucial or should I just be able to throw something there. Like a 2n2222?
Biasing the source to anywhere in the 4.5V +/-1.0V range should be fine.
I'm not against the the BJT buffer version. In many circuits JFET buffers don't add anything other than cost and headaches. It's generally a good practice to use a high-gain low-noise transistor for a buffer. If you use a low gain transistor it can still load the input and the emitter bias voltage can drop.
Quote from: antonis on July 12, 2022, 11:16:45 AM
edit: Just realized how badly it sounds in English.. :icon_redface:
(sorry, no offence intention..)
No offence taken! I thought it maybe is typical Greek humor ;D
I think such wisdom needs to go on a T-shirt. :icon_cool:
Did up the bjt buffer last night got it all set up voltage going into the output cap of it was right at the 3.6. Hooked it all up and the squealing was back no signal. Took it out of the signal path, and still the squeal. I've got some different jfets coming and an386-3. I'll keep tinkering on this and play with the voltage divider and buffer will let you know if I get anywhere. Thanks again for sharing your knowledge.
Quote from: Locrian99 on July 13, 2022, 02:41:48 PM
Did up the bjt buffer last night got it all set up voltage going into the output cap of it was right at the 3.6. Hooked it all up and the squealing was back no signal. Took it out of the signal path, and still the squeal. I've got some different jfets coming and an386-3. I'll keep tinkering on this and play with the voltage divider and buffer will let you know if I get anywhere. Thanks again for sharing your knowledge.
Well, if it's not a LM386 oscillation issue it could very well be a layout or bypassing issue, for example:
The wiring of the volume pot is too close to the speaker output wire. Best is to move the wires away from each other. Beyond that is adding a 10k resistor between the volume wiper and the IC, located at the IC end of the wire. If that doesn't work also add say a 220pF cap from the IC input to ground.
You can also get oscillation problem when the speaker ground currents flow through the same ground tracks as the smaller audio signals. You could try connect the 0V from the 0V near pin 4 of the IC and the ground wire of the speaker back to the sam point. If you have a 6.5mm jack for the speaker, or you are switching the 9V power on and off using the input jack, like an effects pedal does, it can promote bad grounding when you have high current circuits such as an amp driving a speaker.
Yet another possibility is the oscillation is caused by power supply fluctuations feeding back into the buffer. In this case you need to filter the power rail going to the bias divider on the buffer; or filter the supply to the buffer. At first you could just try a 100uF cap across the power rail.
Here's an example of a filtered bias set-up. As used on Ibanez pedals.
(https://i.postimg.cc/ThWxQhG2/ibanez-buffer-filtered-bias.gif) (https://postimg.cc/ThWxQhG2)
The unequal resistors are a tweak to raise the bias voltage on the emitter.
You can increase the filter cap as designed.
The actual part values aren't so important it's the idea behind it.
Yea i feel like it's something being too close but it really seems mostly tied to the volume pot, not sure if it's the nature of the circuit. So I will try the resistor, my bench is pretty crowded so it could be plenty of stuff. I actually took a picture of the layout without the squeak I'll check that and see if it has any clues. Also I wondered if somehow my test box was causing something like you mentioned. I can take that out of the equation easily enough with a 9V and an input jack just wired to it. Unfortunately the only speaker I have to hook it up to at the moment is a home theater speaker I had laying around.
I've got a half built deep blue delay I should finish I may do that while I wait for the other Jfet to arrive. Along with my 3.5 mm jack etc
QuoteYea i feel like it's something being too close but it really seems mostly tied to the volume pot, not sure if it's the nature of the circuit.
You have to draw a line between the cause and the fact the volume control has an effect. Oscillation occurs when the output signal feeds back to the input and that the fed back signal is strong enough to sustain oscillation. The volume control is just a dial that sets how much signal goes back around, backing it off has to stop the oscillation at some point. However, the thing that is coming back can come from any part of the circuit before the volume control upto and including the volume control itself.
The added resistor (and added cap) reduces the amount of signal that gets around the unintentional feedback loop. The unintentional feedback signal, especially that due to wiring, is usually stronger at high frequencies than low frequencies. The added resistor and cap knocks down the level at high frequencies only, frequencies outside audio band. That's where this solution can work.
In severe cases the unintentional feedback signal is both strong and within the audio band. This can happen for example when the signal gets in through the power supply or ground. You can't filter the audio path to prevent this without affecting the audio. It's best to filter the problem signal at the source, ie. on the power rail itself, using the filter bias.
When the buffer wasn't working this automatically blocked or reduced the audio around the unintentional feedback path. It's only now when the buffer is working that a deeper problem has been exposed.
My gut feeling is that you have a ground layout problem. Here is a link to a video I found very instructive:
https://www.youtube.com/watch?v=lBGE5lwbruE&ab_channel=JohnAudioTech (https://www.youtube.com/watch?v=lBGE5lwbruE&ab_channel=JohnAudioTech)
The Beavis breadboard layout assumed you had linked power rails between the 2 power strips - I can't clearly see any links to do it. Also, some cheap breadboards may actually have power strips split in the middle of the run, even though there are red & blue stripes printed the whole length.
It isn't actually illegal to have the JFET buffer with the simple bias in the original scheme, but at 9v supply, it's too sensitive to the variance in the JFET characteristics. If you had 12v or higher, the success rate would improve.
I think it was said that the amp chip here is an LM386-1. The datasheet spec only gives a power rating for a 6v supply (Why? It can run up to 12v) with an 8ohm speaker. I don't think I saw what speaker this build has.
Did someone praise the JRC/NJM386? This one is, I believe, only made to the LM386-4 specs.
Quote from: Clint Eastwood on July 14, 2022, 03:17:38 AM
My gut feeling is that you have a ground layout problem. Here is a link to a video I found very instructive:
https://www.youtube.com/watch?v=lBGE5lwbruE&ab_channel=JohnAudioTech (https://www.youtube.com/watch?v=lBGE5lwbruE&ab_channel=JohnAudioTech)
Hey thanks for the video I suspect this is correct. Right now I have pretty much the beavis board layout with the cap called for in the schematic, and the input obviously going to the right place. My power strips are linked at the output end. I will try and mess with it some later. Bit confused on how to split the ground signal exactly. Was thinking run the ground from pins 3 and 4 to one rail and keep all others coming off the other rail? Will look at some vero layouts and see how those address grounds as the end goal for playing with this is to make it on vero and enclose it. Was wanting to compare it with the noisy cricket to see which I wanted to do but I suppose with the tranny and ic socketed won't be out much if I just do up two veros.
So I watched a couple videos and found a few more articles on this. Could get the oscillation completely gone by essentially separating a few grounds to a rail on the breadboard as shown in the video Clint linked. Although interestingly enough if I put that cap he had between pins 6 and 4 it made it worse no matter what I did. I put it on vero just to see what it would do and I have quite a bit more volume and the distortion is gone with the gain dialed way back. It's going to work for a test bench amp. Curious how it sounds through headphones, waiting on an order with the jacks. I've got some lm386-3s and found some mpf102s as well as j113 to try it in once they get here. Thanks for all the advice guys. Kind of wish I would've just put it on vero a bit ago, but feel like I learned a lot through this.
QuoteCould get the oscillation completely gone by essentially separating a few grounds to a rail on the breadboard as shown in the video Clint linked.
The video does a good job showing what needs to be done (and why ;D).
QuoteAlthough interestingly enough if I put that cap he had between pins 6 and 4 it made it worse no matter what I did.
Yes, that's a bit odd. You might try moving the ground connection side of the cap to be towards the noisy/speaker ground more than the input ground.
If you enclosure is metal and any if more than one of the jacks connect to the enclosure you can get multiple ground connections through the enclosure and that undermines the separate ground scheme (any connectors can do this, audio in, DC power in, speaker output).
QuoteCurious how it sounds through headphones, waiting on an order with the jacks.
It should work fine. Make sure you connect the headphones via a cap not directly to the chip. You can just connect them to the speaker output. Slightly better is to put say 33ohm upto 100ohm in series with each headphone line. What that does is balance the level with different headphones impedances. With a higher value like 100 ohm you lose a bit more level and you have to judge if that's a too much output drop.
Cool. If I want to use a 12v power supply is there anything special I need to do?
Quote from: Locrian99 on July 16, 2022, 03:53:38 AM
Cool. If I want to use a 12v power supply is there anything special I need to do?
Depends on the speaker used. you can easily exceed the power rating of the chip. At 12v supply, 16ohm gets up to 1 watt rms? Headphones would be safer WITH the 100R resistors Rob mentioned. Don't want to deafen you.
Alright sounds good. If I use this set up for the headphones just replace that link from the strip with the output from board and speaker out with a 33-100 ohm resistor?
(https://i.postimg.cc/5X5SYCz2/38-C51221-A5-D3-45-B6-AACB-F4-D3-EF61-AD41.png) (https://postimg.cc/5X5SYCz2)