You're correct Mooly, I made the mod last night and it does seem to work fine.
Here's the circuit:
Whilst listening to some music I connected the phono leads to the JBL Sub6 (with it powered down so no audio from it) and I heard a slight reduction in audio level from the A3i through the Tannoys. To get back up to the same level I had to turn the vol control through only a few (maybe 10) degrees.
So all pretty good on the A3i side, but a couple of things:
I powered up the sub and the sound level coming from it is a little too low. Even with the Sub6 volume control turned up to max, the LF level is somewhere close, but not quite there.
Another problem is the Sub6 gets a bit noisy when its volume control is at max (background hum/buzz) which is annoying during quiet passages. Knock it back a little and the noise reduces significantly.
So I'm considering making another modification; increase the value of R907 in the Sub6 in an attempt to bring the input to its pre-amp higher up in the resistor chain. Does this seem a reasonable thing to do (am I understanding this circuit correctly or barking up the wrong tree)?
Cheers.
Here's the circuit:
Whilst listening to some music I connected the phono leads to the JBL Sub6 (with it powered down so no audio from it) and I heard a slight reduction in audio level from the A3i through the Tannoys. To get back up to the same level I had to turn the vol control through only a few (maybe 10) degrees.
So all pretty good on the A3i side, but a couple of things:
I powered up the sub and the sound level coming from it is a little too low. Even with the Sub6 volume control turned up to max, the LF level is somewhere close, but not quite there.
Another problem is the Sub6 gets a bit noisy when its volume control is at max (background hum/buzz) which is annoying during quiet passages. Knock it back a little and the noise reduces significantly.
So I'm considering making another modification; increase the value of R907 in the Sub6 in an attempt to bring the input to its pre-amp higher up in the resistor chain. Does this seem a reasonable thing to do (am I understanding this circuit correctly or barking up the wrong tree)?
Cheers.
Right... 
It's much easier to visualise when drawn out like that. Hmm. I can see one possible 'issue'. The resistive mixing is going to reduce channel separation as it will slightly mix the audio at the wiper/s of the volume control.
Altering R907 will alter the overall level of the sub. You can go all the way from zero to open circuit with that one to get the level you want.
It's much easier to visualise when drawn out like that. Hmm. I can see one possible 'issue'. The resistive mixing is going to reduce channel separation as it will slightly mix the audio at the wiper/s of the volume control.
Altering R907 will alter the overall level of the sub. You can go all the way from zero to open circuit with that one to get the level you want.
I'm glad the circuit was useful, a picture paints a thousand words and all that. It's amazing what you can knock-up in PowerPoint.
Thanks for the feedback on the sub issue; much appreciated. I apologize it's off this threads subject ("A3i repairs and mods", not Sub6 mods) so I'll play with the resistor value (maybe introduce a variable resistor if possible) and see how it goes.
Thanks, also, for highlighting the issue with channel separation; I take on board what you're saying and must admit I wasn't overstruck on the way the sub's input circuit links the 2 channels together. With this in mind, I've now listened to some familiar tracks with quite noticeable stereo mixing and I can honestly say they seem unchanged and acceptable, certainly to my 60 year old ears . I'm guessing (read hoping) that the channel separation issue gives a problem only to the stereo imaging for the listener and nothing detrimental to the A3i itself, am I assuming correctly? 🤞
On a different subject, whilst the A3i was apart, I noticed a tired looking resistor on the front board which, when poked with my finger, disintegrated into a pile of dust! It was R613, so needless to say the power indicator on the front panel no longer works (I'm surprised it worked beforehand, but it did!). Another job for another day.
Thanks again
Thanks for the feedback on the sub issue; much appreciated. I apologize it's off this threads subject ("A3i repairs and mods", not Sub6 mods) so I'll play with the resistor value (maybe introduce a variable resistor if possible) and see how it goes.
Thanks, also, for highlighting the issue with channel separation; I take on board what you're saying and must admit I wasn't overstruck on the way the sub's input circuit links the 2 channels together. With this in mind, I've now listened to some familiar tracks with quite noticeable stereo mixing and I can honestly say they seem unchanged and acceptable, certainly to my 60 year old ears
. I'm guessing (read hoping) that the channel separation issue gives a problem only to the stereo imaging for the listener and nothing detrimental to the A3i itself, am I assuming correctly? 🤞On a different subject, whilst the A3i was apart, I noticed a tired looking resistor on the front board which, when poked with my finger, disintegrated into a pile of dust! It was R613, so needless to say the power indicator on the front panel no longer works (I'm surprised it worked beforehand, but it did!). Another job for another day.
Thanks again
Channel separation isn't as critical as you might think... back in the day tuners and cassette decks (and vinyl) had very mediocre separation, around 30db I think. We are used now to digital sources and amps with almost no channel interaction.
Strange to say that a little mixing and blending between channels can sometimes be subjectively better sonically according to some listeners.
Ideally the two channels would be mixed via an opamp which would ensure no channel interaction.
The burnt resistor sounds an odd one. I would need to see a circuit, but it takes some doing for what I assume is a series feed resistor to an LED to burn up. Maybe there is something else going on there.
Strange to say that a little mixing and blending between channels can sometimes be subjectively better sonically according to some listeners.
Ideally the two channels would be mixed via an opamp which would ensure no channel interaction.
The burnt resistor sounds an odd one. I would need to see a circuit, but it takes some doing for what I assume is a series feed resistor to an LED to burn up. Maybe there is something else going on there.
Hi,
Just an under-rated resistor that was very old. Consider changing to a new efficient LED and dropping the current when you replace the resistor. I haven't looked up that particular schematic, but I don't think it is a big concern. I have seen this many times before in various equipment. If you keep the same value and indicator, go up a couple sizes in dissipation rating for that resistor. After all, it physically failed so it has been running hot.
Channel sep. for many cartridges ranged from 22 dB up to maybe 27 dB. Many tuners managed 25 dB while other better tuners might top 50 dB. Commonly around 30 ~ 35 dB. Tape decks managed rather better numbers, noise floor was the big issue there (and HF response). Then consider actual separation from your speakers in the room, that number might depress you. It would matter if you were using headphones, but with free standing speakers it often isn't critical. You do need over 30 dB and some setups simply can't even deliver that (acoustically).
-Chris
Just an under-rated resistor that was very old. Consider changing to a new efficient LED and dropping the current when you replace the resistor. I haven't looked up that particular schematic, but I don't think it is a big concern. I have seen this many times before in various equipment. If you keep the same value and indicator, go up a couple sizes in dissipation rating for that resistor. After all, it physically failed so it has been running hot.
Channel sep. for many cartridges ranged from 22 dB up to maybe 27 dB. Many tuners managed 25 dB while other better tuners might top 50 dB. Commonly around 30 ~ 35 dB. Tape decks managed rather better numbers, noise floor was the big issue there (and HF response). Then consider actual separation from your speakers in the room, that number might depress you. It would matter if you were using headphones, but with free standing speakers it often isn't critical. You do need over 30 dB and some setups simply can't even deliver that (acoustically).
-Chris
Hey, thanks for the replies guys, great info which has made me happy with the channel separation issue, I won't be getting too hung up about it, cheers
Edit: I've just played Nancy Sinatra's Bang Bang from the Kill Bill Vol1 CD, where her voice starts fully panned to the right channel in the master mix. I disconnected the right speaker and she's not in the left speaker at all, so well chuffed
Here's the circuit from the bridge to the Power LED via R613:
The LED is a nice blue so would like to keep it if possible, so I'll try replacing R613 with a resistor of higher power rating than the original, then keep an eye on it for a while 🔥 ....
Edit: I've just played Nancy Sinatra's Bang Bang from the Kill Bill Vol1 CD, where her voice starts fully panned to the right channel in the master mix. I disconnected the right speaker and she's not in the left speaker at all, so well chuffed
Here's the circuit from the bridge to the Power LED via R613:
The LED is a nice blue so would like to keep it if possible, so I'll try replacing R613 with a resistor of higher power rating than the original, then keep an eye on it for a while 🔥 ....
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Well, the current is approximately 14 mA with a drop around 32 VDC, that means a dissipation of approximately 460 mW. Running the LED at 14 mA is pretty high and a modern LED can probably be run at a lower current. We're talking blue. Since it is a power indicator you could likely run it around 8 mA and it should be bright enough, maybe too bright with a current production LED. The dissipation would then be around 260 mW, still pretty high. I assumed a 35 V supply by the way.
R502 does nothing. You would be way further ahead to split the dissipation between them by making them equal values. Otherwise R502 is merely a jumper. Using a 34 V assumption, your value would be around 2K0 each resulting in less than 8 mA. The dissipation then falls to around 120 mW each. Much more reasonable!
Hope that helps!
R502 does nothing. You would be way further ahead to split the dissipation between them by making them equal values. Otherwise R502 is merely a jumper. Using a 34 V assumption, your value would be around 2K0 each resulting in less than 8 mA. The dissipation then falls to around 120 mW each. Much more reasonable!
Hope that helps!
30 ohm.
Thinking about it and a modern LED would be super super bright at that current. Even 1ma can be more than enough for subtle panel lighting.
Erm , does this change things any? Sorry I didn't show the full circuit after the 30 Ohm as I thought it wouldn't matter, my bad...
Perhaps I should try a 4k7 instead of the 2k2 and see if that's better. I'm not bothered if it's dimmer than before, so long I can see it's on then it fulfils its purpose in life.
Perhaps I should try a 4k7 instead of the 2k2 and see if that's better. I'm not bothered if it's dimmer than before, so long I can see it's on then it fulfils its purpose in life.
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Hi Karl,
Yes, you're right. I haven't tested a blue LED at 1 mA, and it depends on how bright Redtrev wants it. However at 1 mA your dissipation drops down to 31 mW, so any resistor can handle that with ease. The drop on a blue LED should be around 3 VDC - very approximately. At 2 mA (handy LED tester) it is plenty bright with a clear lens, a diffused might be less bright but more even. I just tested a couple blue 5 mm LEDs.
Hi Redtrev,
Yeah, that kinda changes things. R502 is still basically just a jumper but you can't increase that value without affecting the tone circuits. Mind you, the rest of that circuit must have a filter cap and (hopefully) at least a zener diode to control the supply voltage. Without a filter you are feeding the tone op amp with pulsating DC.
I would definitely replace the power indicator and increase the resistance to allow around 1 ~ 2 mA to flow. That makes the demands on that dropping resistor sane.
Yes, you're right. I haven't tested a blue LED at 1 mA, and it depends on how bright Redtrev wants it. However at 1 mA your dissipation drops down to 31 mW, so any resistor can handle that with ease. The drop on a blue LED should be around 3 VDC - very approximately. At 2 mA (handy LED tester) it is plenty bright with a clear lens, a diffused might be less bright but more even. I just tested a couple blue 5 mm LEDs.
Hi Redtrev,
Yeah, that kinda changes things. R502 is still basically just a jumper but you can't increase that value without affecting the tone circuits. Mind you, the rest of that circuit must have a filter cap and (hopefully) at least a zener diode to control the supply voltage. Without a filter you are feeding the tone op amp with pulsating DC.
I would definitely replace the power indicator and increase the resistance to allow around 1 ~ 2 mA to flow. That makes the demands on that dropping resistor sane.
Haha, yes, this amp is from the 90's. It's old but I like the sound of it, I don't drive it hard. I used to have the sub connected via its High Level (speaker level) inputs but was never happy with the overall sound, it sounded messy. When I used the Tape (record) outputs from the A3i into the line inputs on the Sub6 it cleaned everything up, but I had no single control over the sub level, hence my desire to introduce pre-amp outputs. So far so good with this one.
Now you've mentioned the blue LED relative to the manufacture date, you've got me thinking back. I'm sure I bought this A3i as a reconditioned unit from Richer Sounds in Birmingham. It was still a current model back then. I have a vague memory of the sales rep commenting on the blue LED, suggesting is was a special / sought-after feature. I guess it was just a replacement during its refurb, as you suggest above, to be honest.
Now you've mentioned the blue LED relative to the manufacture date, you've got me thinking back. I'm sure I bought this A3i as a reconditioned unit from Richer Sounds in Birmingham. It was still a current model back then. I have a vague memory of the sales rep commenting on the blue LED, suggesting is was a special / sought-after feature. I guess it was just a replacement during its refurb, as you suggest above, to be honest.
The first blue LED's to appear from the likes of Maplin and RS were very expensive, certainly a fiver each if not more for the first ones while the standard red/green/yellow were around 30p. Now if you want blue or white LED's you can get a string of Christmas lights with hundreds of them for a few pounds.
Ok , that explains, and I guess justifies, the reps enthusiasm for the blue LED. The burnt out resistor was raised by about 15mm off the pcb with, what looks like, a type of fibreglass sleeve around the legs; now whether that was production intent or part of the reconditioning I don't know.