I recently acquired a Quad 306 power amp in excellent condition for free that was going to be thrown out, so of course, I had to take it! It powers on fine, and music plays through it, but the sound is quite muddy and distorted. Not massively, but enough to make it not fit for purpose as it is right now.
I opened it up to have a look, and I couldn't see any components that had obviously burnt out or anything. I did though notice that some modifications and some recapping had been done though. It looks like the 4 main (power supply?) capacitors have been replaced. The speaker inputs have been replaced, and looked like they were wired really badly to the PCB, so I desoldered the wires, and replaced them. This unfortunately did not help with the sound. I used 20AWG wire rated for 600V for this, is this wire fit for purpose?
Just a note, I'm not very experienced with electronics, I have some very rudimentary knowledge and basic soldering and multimeter skills, but I am keen and willing to learn.
On the underside of the board, there are 8 100nf capacitors, film capacitors I think. I have done a bit of research, and I'm pretty certain these are to decrease the input sensitivity, as the Quad 306's sensitivity is 375mV out of the factory. I have attached some photos of this.
Could anyone help me work out by how much exactly this is decreasing the input sensitivity? And could this mod be causing the sound issues? Would desoldering them be ok? I will attach a photo of the top side of the PCB, in case anyone can spot any other changes that might have been made that I've missed. Some of the solder joints on the underside are touching each other, could this cause anything?
Also, what should I do in a situation like this where an amp has a sound issue? I have seen some advice elsewhere that involves sending a sine wave through the amplifier, but I don't think I have the equipment to do this.
Thanks in advance for any help!
EDIT: I think I found some information on the mods that were done. It's a kit from here: https://www.dadaelectronics.eu/uplo...als/Quad-306-Upgrade-Revision-Manual-V1.4.pdf . The resistors at R6 have been changed from 120ohm to 64ohms. I don't think R13 has been changed, so the input sensitivity hasn't been changed I don't think. R9 reads as 27 whereas it should be 33, and it does match the colour for a 27ohm, so this might be incorrectly installed. R11 reads as 120ohm but should be 47ohms, so it looks like this either shouldn't be here as well, or was put in for some reason. R21 reads as 0.01 or 0ohms whereas it should be 22ohm. It looks like the right colour resistor though, as R20 is a 22ohm as well and it's the same colours.
Oh, and I've just realised, if I am correct that the person who did this was following this companies mod, there are only 8 caps fitted on the underside, as opposed to 10 in their manual.
Here are the photos: Quad 306 - Album on Imgur
p.s. The schematic is here Quad 306 Schematic and the manual is here https://www.dadaelectronics.eu/uploads/downloads/02_Quad-Service-Manuals/Quad-306-Service-Manual.pdf
I opened it up to have a look, and I couldn't see any components that had obviously burnt out or anything. I did though notice that some modifications and some recapping had been done though. It looks like the 4 main (power supply?) capacitors have been replaced. The speaker inputs have been replaced, and looked like they were wired really badly to the PCB, so I desoldered the wires, and replaced them. This unfortunately did not help with the sound. I used 20AWG wire rated for 600V for this, is this wire fit for purpose?
Just a note, I'm not very experienced with electronics, I have some very rudimentary knowledge and basic soldering and multimeter skills, but I am keen and willing to learn.
On the underside of the board, there are 8 100nf capacitors, film capacitors I think. I have done a bit of research, and I'm pretty certain these are to decrease the input sensitivity, as the Quad 306's sensitivity is 375mV out of the factory. I have attached some photos of this.
Could anyone help me work out by how much exactly this is decreasing the input sensitivity? And could this mod be causing the sound issues? Would desoldering them be ok? I will attach a photo of the top side of the PCB, in case anyone can spot any other changes that might have been made that I've missed. Some of the solder joints on the underside are touching each other, could this cause anything?
Also, what should I do in a situation like this where an amp has a sound issue? I have seen some advice elsewhere that involves sending a sine wave through the amplifier, but I don't think I have the equipment to do this.
Thanks in advance for any help!
EDIT: I think I found some information on the mods that were done. It's a kit from here: https://www.dadaelectronics.eu/uplo...als/Quad-306-Upgrade-Revision-Manual-V1.4.pdf . The resistors at R6 have been changed from 120ohm to 64ohms. I don't think R13 has been changed, so the input sensitivity hasn't been changed I don't think. R9 reads as 27 whereas it should be 33, and it does match the colour for a 27ohm, so this might be incorrectly installed. R11 reads as 120ohm but should be 47ohms, so it looks like this either shouldn't be here as well, or was put in for some reason. R21 reads as 0.01 or 0ohms whereas it should be 22ohm. It looks like the right colour resistor though, as R20 is a 22ohm as well and it's the same colours.
Oh, and I've just realised, if I am correct that the person who did this was following this companies mod, there are only 8 caps fitted on the underside, as opposed to 10 in their manual.
Here are the photos: Quad 306 - Album on Imgur
p.s. The schematic is here Quad 306 Schematic and the manual is here https://www.dadaelectronics.eu/uploads/downloads/02_Quad-Service-Manuals/Quad-306-Service-Manual.pdf
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The caps look to be only soldered across zener diodes. Zeners are being used as power supply stabilizers so the function of the extra caps is probably to improve (lower) the AC impedance of the power supplies at higher frequencies. I can't see that this'll have any effect on the amp's input sensitivity. If there are wires to the caps which are shorting then best remove the caps.
Welcome to DIYA btw.
<edit> Just wanted to add that to change the input sensitivity it would indeed be resistor values that have to change. Resistors normally determine the gain of a circuit.
Welcome to DIYA btw.
<edit> Just wanted to add that to change the input sensitivity it would indeed be resistor values that have to change. Resistors normally determine the gain of a circuit.
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Thanks for your reply. If I am correct in thinking that the person that fiddled with this before was following these companies mods and instructions (https://www.dadaelectronics.eu/uplo...als/Quad-306-Upgrade-Revision-Manual-V1.4.pdf), then it looks like they did not change the resistors to modify the input sensitivity. I think they put the caps across the zerners to reduce noise, I think. I gathered that from briefly googling what decoupling capacitors across zerner diodes does.
I have ordered some 100nf caps, just in case I might put the two that are missing across C7, like in dada electronics instructions.
I have ordered some 100nf caps, just in case I might put the two that are missing across C7, like in dada electronics instructions.
The 100nFs will reduce noise, but as far as I can see only above the audio band (i.e. >20kHz). If audio band noise reduction is desired then the caps would need to be considerably bigger - perhaps upto 470uF/10V electrolytics. But there may not be room under the PCB for such caps, they'd be 6.3mm or 8mm diameter typically.
R6 is 120K, not 120R. Changing it to 64K, surely 62K if the Dada kit was followed, is unexplained in their instructions, evidently mirroring the AC gain change via R13->18R which wasn't done. R21 is in parallel with L2 so of course it will read very low. The Dada kit bypasses D1-4 and C7 with 100nF caps in the copper side, 5 per channel = 10.
Sorry, yes you're correct, I'm measuring 62K on R6, so it looks like it is the Dada kit being followed. I'm still getting used to using a multimeter. Ah, thank you, that explains R21 then.
R11 reads at about 119R, not 47R like in the manual, it looks like it might be in parallel with R14 (an educated guess, I'm very new to schematics), but something else would have to be affecting it as well I think. Could this be why it gives a different reading?
I'll have some 100nF caps coming in a few days, so I'll add on the two missing caps then.
R11 reads at about 119R, not 47R like in the manual, it looks like it might be in parallel with R14 (an educated guess, I'm very new to schematics), but something else would have to be affecting it as well I think. Could this be why it gives a different reading?
I'll have some 100nF caps coming in a few days, so I'll add on the two missing caps then.
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I would put R6 back to factory value. R11 isn't in parallel with R14 but I would leave it alone, it may have been varied by the factory for stability reasons.
[Being in parallel with R14 would make it read lower, not higher.]
The 100nFs are an RF noise measure, not AF.
[Being in parallel with R14 would make it read lower, not higher.]
The 100nFs are an RF noise measure, not AF.
Ah ok, thanks for explaining that to me!
With regards to what you said about the caps, does that mean the 100nFs are for reducing interference from things external to the amp, and they don't directly affect the sound from it?
I have taken off the 8 100nF caps for now, so I can start from a clean slate, and will eventually put 10 back in as per the Dada instructions.
Now I think the only thing which isn't stock on it is the resistor at R6 and I'll get round to replacing that tomorrow. It still sounds quite distorted, so hopefully that helps.
With regards to what you said about the caps, does that mean the 100nFs are for reducing interference from things external to the amp, and they don't directly affect the sound from it?
I have taken off the 8 100nF caps for now, so I can start from a clean slate, and will eventually put 10 back in as per the Dada instructions.
Now I think the only thing which isn't stock on it is the resistor at R6 and I'll get round to replacing that tomorrow. It still sounds quite distorted, so hopefully that helps.
You haven't said what ancillary equipment you are using... (you say you only have a multimeter so I guess you're not testing with a signal generator)
What are you feeding it from and what speakers are you using..
What are you feeding it from and what speakers are you using..
I only have a multimeter, no signal generator, unfortunately. I've been feeding it from a few different headphone amps that have pre outs, a Schiit Vali 2 and a Drop THX aaa one, and a cheap Chinese tube preamp kit.
My audio source is a TT into a Tricord Dino phono stage. I first tested it on some cheap 70's speakers, but I've also tested it on my KEF Q55s once it seemed fairly safe to do so.
With all the pres I tied, the sound was a bit distorted and muddy, particularly the highs I think.
I might possibly pick up a cheap signal generator from Amazon to start off with, maybe something like this FG-100 DDS Function Signal Generator, 1HZ-500KHZ Low Frequency Counter Signal Source Module, Main Output Waveform are Sine Wave/Square Shaft/Triangular Shaft/Sawtooth Shaft : Amazon.co.uk: Business, Industry & Science . But I think I'll look for one with better reviews. Almost certainly won't be great, but it's somewhere to start.
My audio source is a TT into a Tricord Dino phono stage. I first tested it on some cheap 70's speakers, but I've also tested it on my KEF Q55s once it seemed fairly safe to do so.
With all the pres I tied, the sound was a bit distorted and muddy, particularly the highs I think.
I might possibly pick up a cheap signal generator from Amazon to start off with, maybe something like this FG-100 DDS Function Signal Generator, 1HZ-500KHZ Low Frequency Counter Signal Source Module, Main Output Waveform are Sine Wave/Square Shaft/Triangular Shaft/Sawtooth Shaft : Amazon.co.uk: Business, Industry & Science . But I think I'll look for one with better reviews. Almost certainly won't be great, but it's somewhere to start.
There are several DC voltages on your schematic. I would start out measuring those, before jumping in and changing film caps. Film caps are usually 100 year life parts unless the solder joints are bad.
In particular R18 2 places, DZ2 one place, D3 one place, T6 3 places.
Warning to newbies, don't use 2 hands working on an appliance with the power on. Voltage >25 across the heart can stop it. No jewelry on hands wrists or neck, current through metal can burn your flesh to charcoal. Wear safety glasses, solder splashes and parts explode. put the negative DVM probe to the analog ground with an alligator clip lead with one hand, then probe voltages with the red with the same hand.
When probiing defective amps I use a throwaway car radio speaker from the junkyard, of the proper impedance, usually 8 ohms. 10 is okay. You can put back to back ~1000 uf electrolytic caps in series with it to protect even it against DC, if you short something. Back to back electrolytics sound a little funny at low wattage, but not REAL bad.
I don't obsess about a signal generator, I use earphone jack of a $5 battery FM radio, turned down to about 1/4 volume. Full volume is 7 vac, too much. With a amp sensitivity of 375 mv, you may want to turn radio down to 1/8 volume. Check station tuning with earphone before proceeding to put music into the amp with 1/8 stereo phone plug to dual RCA plug adapter cable.
In particular R18 2 places, DZ2 one place, D3 one place, T6 3 places.
Warning to newbies, don't use 2 hands working on an appliance with the power on. Voltage >25 across the heart can stop it. No jewelry on hands wrists or neck, current through metal can burn your flesh to charcoal. Wear safety glasses, solder splashes and parts explode. put the negative DVM probe to the analog ground with an alligator clip lead with one hand, then probe voltages with the red with the same hand.
When probiing defective amps I use a throwaway car radio speaker from the junkyard, of the proper impedance, usually 8 ohms. 10 is okay. You can put back to back ~1000 uf electrolytic caps in series with it to protect even it against DC, if you short something. Back to back electrolytics sound a little funny at low wattage, but not REAL bad.
I don't obsess about a signal generator, I use earphone jack of a $5 battery FM radio, turned down to about 1/4 volume. Full volume is 7 vac, too much. With a amp sensitivity of 375 mv, you may want to turn radio down to 1/8 volume. Check station tuning with earphone before proceeding to put music into the amp with 1/8 stereo phone plug to dual RCA plug adapter cable.
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Thanks, I'll take a look at those voltages tomorrow. And I'll take heed of those safety tips. I built a Bottlehead Crack headphone amp recently, so I learnt a lot of those cardinal rules from there, also. I'll stick to using my cheap/backup pair of speakers for testing then.
When std these have quite a high input sensitivity, in the region of 0.3v IIRC which is quite a bit lower than most modern stuff, but if you was driving it into clipping it would be LOUD.
One thing I've done in the past is a cheap powered PC computer speaker with the input swapped to a BNC socket. I then use a std oscilloscope probe, connect the ground to something grounded on the amp (RCA input surround usually with plug and clip to the ground tag on that) and probe my way through the amp from input to power stage, just listening to the audio/signal to check for distortion or where the circuit goes dead. This is very basic but I've successfully found where a signal is lost or becomes distorted in a circuit with it many times.
Hope you get it working, but PM'd you in case you dont!
One thing I've done in the past is a cheap powered PC computer speaker with the input swapped to a BNC socket. I then use a std oscilloscope probe, connect the ground to something grounded on the amp (RCA input surround usually with plug and clip to the ground tag on that) and probe my way through the amp from input to power stage, just listening to the audio/signal to check for distortion or where the circuit goes dead. This is very basic but I've successfully found where a signal is lost or becomes distorted in a circuit with it many times.
Hope you get it working, but PM'd you in case you dont!
I'm either going to change the input sensitivity to 775mV by replacing R13 with an 18R resistor, or to 1V by replacing it with a 27R resistor. I don't think it was clipping, as I had the volume on the preamp barely up at all.
I've just bought a cheap oscilloscope, would you mind taking a quick look at this and see if it's fit for purpose?Digital Oscilloscope Kit, DSO150 Handheld DIY Oscilloscope with BNC-Clip Cable Probe for Secondary Development, 1 MSa/s 200 KHz Bandwidth : Amazon.co.uk: Business, Industry & Science
I had a quick look at what I might need and this seems to do it.
Would I be able to hook up cheap powered speakers to that scope? Would I just need to wire the speakers to the signal output tab at the top of the device?
Thanks for the help!
I've just bought a cheap oscilloscope, would you mind taking a quick look at this and see if it's fit for purpose?Digital Oscilloscope Kit, DSO150 Handheld DIY Oscilloscope with BNC-Clip Cable Probe for Secondary Development, 1 MSa/s 200 KHz Bandwidth : Amazon.co.uk: Business, Industry & Science
I had a quick look at what I might need and this seems to do it.
Would I be able to hook up cheap powered speakers to that scope? Would I just need to wire the speakers to the signal output tab at the top of the device?
Thanks for the help!
With a scope, you don't need a sound probe. which is what powered speakers are.
You may not recognize distortion on a scope trace. That is what a sound probe is useful for.
No specification for max voltage on the scope listing. Some inexpensive scopes are limited to 80 v dc, which is okay for this low powered amp but not for PA amps.
You may not recognize distortion on a scope trace. That is what a sound probe is useful for.
No specification for max voltage on the scope listing. Some inexpensive scopes are limited to 80 v dc, which is okay for this low powered amp but not for PA amps.
Do you have a cheap powered speaker you could (ab)use for this? What connection does it have for audio?
The sope I bought is rated for 50Vpk. According to the quad service manual, the supply rails are approx 36-41V DC, and the highest voltage displayed on the schematic at common places to measure is 40.6V, so that should be okay, right?
I'll have a look in the garage to see if I have anything laying around, but if I don't, what would be the best input type if I had to buy some cheap ones?
Inductive speakers at full power can drive the output higher than the supply rail. This schematic has D9 to clamp the speaker to the + rail but I don't see a complement to clamp the speaker to the - rail. Don't look at the output at full power with the scope.
Most amplified computer speakers have a stereo 1/8" phone plug for input: To plug into the green earphone jack on the computer. Getting from that to alligator clip leads or pamona grabbers requires some adapting. Best to protect the input of a sound probe from DC voltage with a .047 uf >100 v capacitor series.
Most amplified computer speakers have a stereo 1/8" phone plug for input: To plug into the green earphone jack on the computer. Getting from that to alligator clip leads or pamona grabbers requires some adapting. Best to protect the input of a sound probe from DC voltage with a .047 uf >100 v capacitor series.