[yazawa]

I'm building Bugle and power supply. I have never had built any electronics device before. It's fun. I'm enjoying a lot.

I'm almost done. I'm testing it right now before putting it in a case. A problem I have is static coming from Bugle when I turn a volume to halfway and up. I don't hear hum. Just constant static-y sound. I had a humming problem when the wires to the input RCA jacks were long. I shortened the wires and the problem was now gone.

I can hear the static with or without having the TT connected to Bugle. I use stock parts recommended in the Bugle kit manual. What would be the cause of the problem? What should I look for?

Thanks.
SY

[hagtech]

You likely have a bad wiring connection somewhere.

jh

[yazawa]

Ah, OK. I certainly have bad wiring connections. Since I'm testing Bugle outside a case, I can't really solder RCA jacks to the wires. So I just wrap a wire around a RCA terminal. 

I plan to replace the input and output wires with shielded ones. I will solder up everything and see if the static stops. I will report back to the forum.

Thanks for your help, Jim!

SY

[yazawa]

I finally soldered all wires nicely (I hope). I don't see any obvious cold joints anywhere. But I still hear the same static. Would it be possible that some damaged parts cause the static? Opamps should be fine. I didn't have them on the board when I solder anything on the board. But I might have damaged other parts due to my poor soldering skill.

The static is heard when the volume is about half way up. I don't listen to the music that loud. So it's practically a non issue to me. But my concern is that I might compromise the Bugle's capability.

SY

[poty]

Static and hum noise are most difficult things to overcome if present, but eventually they can be eliminated.
There are 2 possible causes:
-cold joint somewhere on the board (or wires going to it);
-external influence.
If it is possible - could you take some photos of your layout, sometimes it helps?
First of all - short-circuit the R17 and R35 to the ground by a short wire to eliminate input pollution.
Second - try to power Bugle from batteries to eliminate power supply problems (do not connect the power supply to mains).
If you have DIP sockets soldered for opamps you can try to pull the opamps out one at a time, beginning from U1. If after pulling out certain opamp the static stops - the problem is before or in the stage. Then you should inspect the parts more carefully in the suspected area.
If you have soldered the opamps directly to the board you can use a decent value capacitor (around 0.47-1 uF) to short-circuit outputs of each stage to the ground. The result should be equal to pulling out corresponding opamp just before the short-circuited output.
The above steps only help to understand the cause, eliminating the problem is another task.

[Speedskater]

It could be a DC on the volume control problem.
Check to see if there is any DC on the Bugle audio output.
Also check to see if the control unit has AC coupled or DC inputs.

[yazawa]

Thanks Poty for your advice. I did some tests following your suggestions.

First of all, here is my Bugles's status.

• If I connect Bugle to the main amp with no TT connected, I get static + hum when I turn the main amp's volume half way up.
• If I connect TT to Bugle, hum is gone. I get only static.

Should I be concerned about the hum when TT is not connected, or can I safely ignore it since I don't get hum under a normal operation.

On to the testing, I started from the easy ones. I turn off the Bugle PS, pulled the power code out of the case, and connected two 9V batteries to Bugle. I still hear the static. I guess the PS should be fine.

I then, removed opamps form DIP sockets.

• Removed only U1 - The static is reduced slightly, but not much difference.
• Removed only U2 - The static is dramatically reduced. I can hear only a little of it when I turn the volume to the max.
• Removed only U3 - I don't hear anything, even with the volume at the max.

Does this mean that the second stage is the culprit? Should I inspect the parts and joints starting from R28 and up to C4 on the right channel and the equivalent on the left channel?

I will try short-circuiting R17 and R35 today or maybe tomorrow. So is what this means to replace those registers with just wires? Are these registers essentially not necessary? Sorry for my ignorance.

Here are some photos. Let me know if you need to see more, or from different angles.







SY

[yazawa]

Hi Speedskater,

Thanks for your comment. I measured the DC voltage between the points where the wires are soldered on an output RCA jack. Is this a right way to measure? I didn't detect any DC voltage on either jack.

As for the second point, since I have no electronics background, I'm unable to understand what you said. I'd appreciate it if you could elaborate as if I'm a third grader. 

SY

[poty]

Should I be concerned about the hum when TT is not connected, or can I safely ignore it since I don't get hum under a normal operation?

It may be important or not. The hum without TT connected is an indication of some input shielding problems. You have the power supply with a lot of wires with AC in the same case. In my opinion - it's better to have either very short input wires (for example by rotating the PCB) or shielded ones. To eliminate the input pollution in the further tests you can solder a wire in place of R16 and R34 and do not connect the TT at all until you spot the source of the static noise.
Before going further, please check for static after the wires are in place. If possible, just in case - use batteries for the period of checking.

... I turn off the Bugle PS, pulled the power code out of the case, and connected two 9V batteries to Bugle. I still hear the static. I guess the PS should be fine.

I think - yes, you are correct.

... removed opamps form DIP sockets.

• Removed only U1 - The static is reduced slightly, but not much difference.
• Removed only U2 - The static is dramatically reduced. I can hear only a little of it when I turn the volume to the max.
• Removed only U3 - I don't hear anything, even with the volume at the max.

Does this mean that the second stage is the culprit? Should I inspect the parts and joints starting from R28 and up to C4 on the right channel and the equivalent on the left channel?

Practically - yes. But the order of the checks in my view should be the following:
- use Speedskater's idea of presence a DC offset somewhere in the circuit: the first and the second stages are DC coupled, so it is possible that you have some DC after the first stage affecting the second stage. So - check the DC voltages on the pin 1 (to ground) and pin 7 (to ground). They should be zero or very close to zero.
- you have the problem in both channels. It is likely that the source should be in circuits which are shared by both channels. For the second stage it is power supply circuit (pin 8 and 4). You should check this first (cold joints, bad contact in the DIP socket, C8, C11).
- as far as you don't have the noise after removing U2 the most probable place for the problem (if the previous two checks dont' reveal anything suspicious) - R28, R32, R29, C6 (and corresponding parts in another channel). The second place - R21, R22 (R3, R4). I'd first try to (U1 removed, U2, U3 in place) short-circuit the point where R10, R11, R14 connected together to the ground and listen to changes.

I will try short-circuiting R17 and R35 today or maybe tomorrow. So is what this means to replace those registers with just wires? Are these registers essentially not necessary?

The resistors are nessesary! The easier way of short-circuiting the resistors has been written above.

By the way - very good looking build!

P.S. Speedskater wanted to know if your amplifier has caps on the input (AC coupling) or not (DC coupling).

[hagtech]

The input RCA jacks should be insulated from chassis.  Hard to tell from the photo.

jh

[yazawa]

Hi Poty,

Thanks so much for your help. I apologize that I couldn't respond sooner. I finally had some time to experiment a bit.

First, I soldered wires in place of R16 and R34. I still hear the static. So the source of the problem should be in the circuit.

Then, I measured the DC voltage on the pin 1 and pin 7 of U1 and U2. U1 had 2mV and U2 had 51mV. 51mV must be a little too high. I read somewhere that up to only 20mV is acceptable.

I re-flowed the solder on all joints that look even slightly dubious. No change.

Then I removed C8 and C11. The voltage on U2 dropped to 13mV. But the static was still there. I also removed C7 and C10. No Change. I read somewhere that these 4 caps are not necessary when you use Bugle PS. If that's the case, I'd like to leave them removed. Is this OK?

That was the end of experiment today. Now I'm thinking that I might have damaged C3, C6 and other caps, considering that the removal of C8 and C11 brought the DC offset lower. Are these film caps sensitive to heat? I remember that I had a harder time to solder them than registers for some reason. I must have heated them up more that I should.

Jim - RCA jacks are insulated. I put plastic washers between case and jacks. Indeed, it's hard to see on the photo.

SY

[poty]

Hello,
Did you do the measurements with batteries or PS?

First, I soldered wires in place of R16 and R34. I still hear the static. So the source of the problem should be in the circuit.

Mainly - yes, but it may be external EMI too.

Then, I measured the DC voltage on the pin 1 and pin 7 of U1 and U2. U1 had 2mV and U2 had 51mV. 51mV must be a little too high. I read somewhere that up to only 20mV is acceptable.

The U2 stage has amplification of 11, so it can be the result of the previous stage DC offset.

Then I removed C8 and C11. The voltage on U2 dropped to 13mV. But the static was still there. I also removed C7 and C10. No Change. I read somewhere that these 4 caps are not necessary when you use Bugle PS. If that's the case, I'd like to leave them removed. Is this OK?

Strange thing with C8 and C11. In my opinion such huge change should mean something wrong with power lines. Could you measure the voltages on pins 8 (to ground) and 4 (to ground) on each Ux? The offset and the strange thing with removing C8 amd C11 should be a result of a problem with one of positive or negative rail.
You do not need C7, C8, C10, C11 in case of using power supply, but C8, C11 I'd personally left in place - one more "stage" of HF filtering is not bad at all.

...I'm thinking that I might have damaged C3, C6 and other caps, considering that the removal of C8 and C11 brought the DC offset lower. Are these film caps sensitive to heat? I remember that I had a harder time to solder them than registers for some reason. I must have heated them up more that I should.

Don't haste... All parts are sensitive to heat, but you should think about bad things last of all.
Let's try to do "(U1 removed, U2, U3 in place) short-circuit the point where R10, R11, R14 connected together to the ground and listen to changes".

Jim - RCA jacks are insulated. I put plastic washers between case and jacks. Indeed, it's hard to see on the photo.

I'd check this point one more time.

[yazawa]

Hi Poty,

I just realized that I completely misunderstood about the DC offset measurement. I measured the voltage between pin1 and pin7 of opamps, that is I attached a probe of a multimeter to pin1 and another probe to pin7. I should have measured the voltage between a pin and ground. Correct? Like I said, I'm a complete newbie. I'm terribly sorry about this.

I re-measured the DC offset. I attached a red probe of a multimeter to a pin of an opamp and a black probe to the ground post of the pcb board. Here are the results:

U1 pin 1 - 1 mV
U1 pin 7 - 3 mV
U2 pin 1 - 23 mV
u2 pin 7 - 34 mV

I measured this with both PS and batteries and the results were about the same.

I measured the voltage of pin 4 to ground and pin 8 to ground on each opamp. All six measurements are consistently 14.85 volt. I measured this only with PS.

Then, I tried "(U1 removed, U2, U3 in place) short-circuit the point where R10, R11, R14 connected together to the ground and listen to changes". I used a wire with small alligator clips on both ends to connect the left side of R11 (left when you hold the board the output side upward) and the ground post. Am I doing this right? I hear hum in addition to static on the left channel while I hear only static on the right channel. I actually got the same result whether I removed U1 or not.

SY

[poty]

I just realized that I completely misunderstood about the DC offset measurement.

It happens!

U1 pin 1 - 1 mV; U1 pin 7 - 3 mV; U2 pin 1 - 23 mV; u2 pin 7 - 34 mV

U2 is DC coupled to the output of U1, so I think the digits looks fine, maybe 3 mV of pin 7 of U1 is slightly big, but I think you do not have such sensitive and precise multimeter to have very small errors in the range.

I measured the voltage of pin 4 to ground and pin 8 to ground on each opamp. All six measurements are consistently 14.85 volt. I measured this only with PS.

OK.

Then, I tried ...[ short-circuit input of U2 ]. I used a wire with small alligator clips on both ends to connect the left side of R11 (left when you hold the board the output side upward) and the ground post. Am I doing this right?

Yes.

I hear hum in addition to static on the left channel while I hear only static on the right channel. I actually got the same result whether I removed U1 or not.

Hum is normal as soon as we have almost an antenna in the very sensitive input of U2. I usually use as short wire as possible in such testing. The static which stays here in the left channel is more interesting. "Clear" imitations of the previous stages almost impossible without signal generators, but if it is possible, could you make some more checks?
I'd like to know the amount of static when you:
- short-circuit R4 (put a wire or pinchers between the resistor's leds);
- short-circuit the connection point of R8, C1, R13 to the ground.

[yazawa]

Hum is normal as soon as we have almost an antenna in the very sensitive input of U2. I usually use as short wire as possible in such testing.

OK. So I tried again to short-circuit the U2 input using a short wire. Hum is gone, but the amount of static is the same in both channels.

I'd like to know the amount of static when you:
- short-circuit R4 (put a wire or pinchers between the resistor's leds);
- short-circuit the connection point of R8, C1, R13 to the ground.

On both cases, hum is reduced to about a half in the left channel. Getting interesting. What does this tell us?

SY

[poty]

... short-circuit the U2 input ... the amount of static is the same in both channels.
...short-circuit R4, U2 output... On both cases, hum is reduced to about a half in the left channel.

Hum or static?
If all done right... It means the positive input side of U2 is clear. The same result in both cases means that the negative side seems also clear. The fact that the static is still here (but in smaller amount) after eliminating the output of U2 let me think that there could be something wrong either with input circuits of U3 or power supply. The U2 has a side effect on the noise, maybe amplifying, as far as pulling out U2 almost eliminates the static.
What to do next... Let's repeat the testing on R15 and R2. I'd also try to add (solder) a pair of capacitors in the range of 100-500 uF directly to pin8-ground and pin4-ground of U2 (short wires or back side of the board). It's strange for me that so small changes in power rails had so big impact on opamps output.

[yazawa]

Hum or static?

Oh, sorry. Static.

I short-circuit'ed the point where R15 and R9 connect to ground. Static in the left channel is almost gone. I hear a little, very little when I turn the volume to the max. But I'm not so picky to care about that.

I short-circuit'ed R2. That also eliminates static, but it's a little louder than R15. I would say 85% of static is gone.

I'll try adding caps when I get them.

SY

[poty]

I short-circuit'ed the point where R15 and R9 connect to ground. Static in the left channel is almost gone. ...
I short-circuit'ed R2. That also eliminates static, but it's a little louder than R15. I would say 85% of static is gone.

OK! We have symptoms and should decide what illness they are representing.
In my opinion U3 works as intended and doesn't add to the static (blocking signal from the "+" side eliminates anything, R2-test just lowers the amplification of the stage from 10 to 1 - then lowers the amplification of noise coming to the "+" side ten times as intended, very close to your estimation of 85% ).
The same "trick" with input of U2 revealed that the noise is not related to the input side of U2. At the same time blocking everything from the output of U2 helped only half-way. What is worrying me most in the situation - the problem is in both channels. Either you've made a symmetrical mistake (including bad parts), or you have something influencing outside of the board just in the area of the U2 output (wires, transformers, ...).
Thinking straight: the suspicious parts are C1(C4), R9(R27), R15(R33). The first two parts' pairs can be eliminated (for testing only!) easily by short-circuiting them (could be done on the single channel for testing). Be cautious, having 34 mV from the U2 you'll have about 270 mV DC on the output of U3.
The latter (if the previuos test won't reveal anything) should be tougher. Maybe unsoldering?

[yazawa]

I short-circuited C1 and R9 (not at the same time). Static is still there. I haven't removed R15 yet, but I doubt that R15 is damaged. Digikey sent me wrong registers for R15, R17, etc (47.0 k instead of 47.5 k). I didn't notice that until I soldered them. Then I replaced them with ones with right value. I get the same static before and after replacing them.

Then, I short-circuited the point between C1 and R9 to ground. Static is gone! Considering the fact that only half of static is gone when short-circuiting the other side of C1 to ground, then C1 is a culprit? But static didn't change when C1 is shoft-circuited (connect the two leads of C1 with a wire). So I'm not sure how I can interpret this.

SY

[poty]

Then, I short-circuited the point between C1 and R9 to ground. Static is gone! Considering the fact that only half of static is gone when short-circuiting the other side of C1 to ground, then C1 is a culprit? But static didn't change when C1 is shoft-circuited (connect the two leads of C1 with a wire). So I'm not sure how I can interpret this.

The last short-circuiting affect both C1 (you just eliminated everything from it) and R15 (you connect 1k resistor in parallel with it), so I'm unsure definitely what is the culprit. You have resoldered the R15, so it is possible that you have either bad solder joint or damaged traces (there was already a case around that possibility earlier in the forum). It is also possible that the problem is in the inputs of the U3 opamp itself. Looking at the pictures you sent in the forum - the U3 is different from the U1 and U2 in the way it is connected (by adapter). Have you tried to change the position of the opamp?
I should admit that shorting C1 is not "clear" try, because you allow DC from the previous stage to enter the input circuit of the U3. Maybe better way is unsoldering the C1 completely, but I tried to avoid this as much as possible taking into account the problems you have with the caps to solder in place.