Mystery amp schematic - assistance/comments/guesses/insights welcomed
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a.palfreyman
pfm Member
Mouser:
"Vishay 1000 uF Axial 40 VDC Aluminium Electrolytic Capacitors - Axial Leaded – Mouser United Kingdom" https://www.mouser.co.uk/c/passive-components/capacitors/aluminum-electrolytic-capacitors/?capacitance=1000 uF&m=Vishay&termination style=Axial&voltage rating dc=40 VDC
Mr Bargain whom I've used before with no problems:
"5x Philips 1000uf 40v vintage look axial electrolytic capacitor 2222-032-17102 | eBay" https://www.ebay.co.uk/itm/40334026...d=link&campid=5338728743&toolid=20001&mkevt=1
"Vishay 1000 uF Axial 40 VDC Aluminium Electrolytic Capacitors - Axial Leaded – Mouser United Kingdom" https://www.mouser.co.uk/c/passive-components/capacitors/aluminum-electrolytic-capacitors/?capacitance=1000 uF&m=Vishay&termination style=Axial&voltage rating dc=40 VDC
Mr Bargain whom I've used before with no problems:
"5x Philips 1000uf 40v vintage look axial electrolytic capacitor 2222-032-17102 | eBay" https://www.ebay.co.uk/itm/40334026...d=link&campid=5338728743&toolid=20001&mkevt=1
This site contains affiliate links for which pink fish media may be compensated.
OK. An hour and a half in the workshop (don't laugh).
I managed to get 3 axial replacements for the 47µF caps in the front end of the power amp, so they were easy.
The 2 power smoothers in the pre-amp/tone (220µF and 100µF) both became 220µF when replaced, and the original axials were small enough that the radial capacitor leads "doing the splits" spanned the gap OK.
But the main smoothers were quite large (long?). I mounted the new axials by drilling an extra hole for each cap in the board (near the earth end), and running the radial leads through. The radial earth leads were simply soldered to the PCB pads, but the other ends wouldn't reach - and in any case, there was at least one PCB track to cross "ideally" without shorting.
So in 1 case I just use some spare lead trimmings to connect pad-to-lead, and in the track-crossing case I used a short piece of insulated lead, all on the track side of the board.
Not too ugly in the end, but this is the first recap I've done myself, so I was stressing over details.
Axe to Rad by plybench, on Flickr
The +ve input to OA2 is now 1-2mV and the output is 28mV.
I managed to get 3 axial replacements for the 47µF caps in the front end of the power amp, so they were easy.
The 2 power smoothers in the pre-amp/tone (220µF and 100µF) both became 220µF when replaced, and the original axials were small enough that the radial capacitor leads "doing the splits" spanned the gap OK.
But the main smoothers were quite large (long?). I mounted the new axials by drilling an extra hole for each cap in the board (near the earth end), and running the radial leads through. The radial earth leads were simply soldered to the PCB pads, but the other ends wouldn't reach - and in any case, there was at least one PCB track to cross "ideally" without shorting.
So in 1 case I just use some spare lead trimmings to connect pad-to-lead, and in the track-crossing case I used a short piece of insulated lead, all on the track side of the board.
Not too ugly in the end, but this is the first recap I've done myself, so I was stressing over details.
Axe to Rad by plybench, on FlickrThe +ve input to OA2 is now 1-2mV and the output is 28mV.
Last edited:
a.palfreyman
pfm Member
...pretty much exactly as before. Looks like it could be a problem with circuit layout...
Front panel is riveted to transformer frame. Is this a good electrical connection?
Front panel is riveted to transformer frame. Is this a good electrical connection?
OK, I've prodded around with my multimeter, and found some good connections as follows:
PCB gnd is NOT connected to mains earth.
There is no measurable ripple between virtual ground and PCB-gnd, either at the smoothing cap end, or right at the output transistor end.
- mains earth to transformer
- transformer to rivet
- rivet to panel
- all control pots to each other
- (through check) mains earth to control pots
- centre tap secondary to PCB gnd
PCB gnd is NOT connected to mains earth.
There is no measurable ripple between virtual ground and PCB-gnd, either at the smoothing cap end, or right at the output transistor end.
a.palfreyman
pfm Member
Good, at least the front panel and pot bodies are connected to safety earth, even if through a rivet. It ought to be a proper 'wired' connection, but there you go.
Can you measure if there is any ripple between one of the pot bodies (ie safety earth) and: A, the circuit virtual earth; and B, the circuit 0V plane? It has to be getting into the circuit somehow... Probably induced into a high impedance node, as you've already proved by moving the earth at the input from #1 to #2 to #3. This reduces noise at output of oa2 as you reduce input impedance.
Can you measure if there is any ripple between one of the pot bodies (ie safety earth) and: A, the circuit virtual earth; and B, the circuit 0V plane? It has to be getting into the circuit somehow... Probably induced into a high impedance node, as you've already proved by moving the earth at the input from #1 to #2 to #3. This reduces noise at output of oa2 as you reduce input impedance.
Here we go.
- safety-gnd -> virtual-gnd: 6.2V, 50 Hz
- safety-gnd -> PCB-gnd: 4.7V, 50 Hz
- PCB-gnd->virtual-gnd, smoother cap end: not measurable
- PCB-gnd->virtual-gnd, output transistor end: 0 or 1 mV, frequency not measurable
a.palfreyman
pfm Member
No, not unusual. When I was playing with my AR XA into a DIY MM phono stage (earth wire of tt connected to case of phono stage, powered by 35Vdc printer PSU > ALWSR, circuit 0V connected to case) and that connected to my DIY headphone amp, I got a slight hum. Headphone amp case has safety earth, but nowhere was the metalwork of the tt or the phono amp connected to mains earth. Measuring between tt arm and headphone amp case gave around 12Vdc... Adding a connection from the earth point on the MM phono stage to the headphone amp case (ie to safety earth) cured the hum.
It suggests that your circuit is floating. I wonder if the input connector body (and therefore screen) should be connected to the case / safety earth? Could do with some input from someone technical here TBH.
It suggests that your circuit is floating. I wonder if the input connector body (and therefore screen) should be connected to the case / safety earth? Could do with some input from someone technical here TBH.
a.palfreyman
pfm Member
Shamelessly stolen from Elliott Sound Products website:
'The connections shown are very similar (ok, virtually identical) to those used in my prototype. Noise is low, but when used at full gain (maximum volume, master gain set to desired level) there will always be some noise - high gain comes with noise (hiss), and it's not possible to have very high gain and no audible noise. All connectors must be fully insulated types, so there is no connection to chassis. This is very important!
You will see from the above diagram that I did not include the 'loop breaker' circuit shown in the power supply diagram. For my needs, it's not required, for your needs, I shall let you decide. If you choose to use it, then the earth (chassis) connection marked * (next to the input connectors) must be left off, and replaced with a 100Ω resistor with a 100nF capacitor in parallel. This is usually enough to prevent earth/ ground loop hum.'
Link to page here: "40W Guitar Amplifier (P27)" https://sound-au.com/project215-p27-revisit.htm
'The connections shown are very similar (ok, virtually identical) to those used in my prototype. Noise is low, but when used at full gain (maximum volume, master gain set to desired level) there will always be some noise - high gain comes with noise (hiss), and it's not possible to have very high gain and no audible noise. All connectors must be fully insulated types, so there is no connection to chassis. This is very important!
You will see from the above diagram that I did not include the 'loop breaker' circuit shown in the power supply diagram. For my needs, it's not required, for your needs, I shall let you decide. If you choose to use it, then the earth (chassis) connection marked * (next to the input connectors) must be left off, and replaced with a 100Ω resistor with a 100nF capacitor in parallel. This is usually enough to prevent earth/ ground loop hum.'
Link to page here: "40W Guitar Amplifier (P27)" https://sound-au.com/project215-p27-revisit.htm
a.palfreyman
pfm Member
What he's saying as far as I interpret it is that a guitar amp should not have the 0V plane connected the the chassis/safety earth because of the probability of hum loops with other gear where these might already have 0V plane and safety earth connected. So, connect 0V plane to chassis/safety earth through a 100R parallel with 0.1uF which is a typical 'loop breaker'.
a.palfreyman
pfm Member
I'd guess it's about on the limit, but it is a guess as for HiFi 50mV is usually accepted as the desired limit.
It amounts to 20-25mA for 4-6ohm, which I guess will be the DC resistance of the coil.
Use it and enjoy it and see how it settles... I'd just recheck it in a months time.
It amounts to 20-25mA for 4-6ohm, which I guess will be the DC resistance of the coil.
Use it and enjoy it and see how it settles... I'd just recheck it in a months time.
Getting access to the PCB involves removing the main plug, desoldering the speaker leads, remove the speaker panel (6 screws), and finally removing the circuit (4 screws).
So, while I've got the access I measured (top/upper bottom/lower are in terms of the schematic)
Top of the bias diode pair (base of BFT82): 0.718V
Bottom of the bias diode pair (base of BFR61): -0.693V
Base resistor of upper BD711: base:101mV, o/p -98mV (so the DC bias is actually negative)
Base resistor of lower BD711: base -34.8V, lower (negative pwr rail) -35.0V.
So both 100Ω resistors have around 200mV across them, current = 2mA
DC resistance of the speaker (currently unwired) is 6.8Ω (nominal is 8Ω)
So, while I've got the access I measured (top/upper bottom/lower are in terms of the schematic)
Top of the bias diode pair (base of BFT82): 0.718V
Bottom of the bias diode pair (base of BFR61): -0.693V
Base resistor of upper BD711: base:101mV, o/p -98mV (so the DC bias is actually negative)
Base resistor of lower BD711: base -34.8V, lower (negative pwr rail) -35.0V.
So both 100Ω resistors have around 200mV across them, current = 2mA
DC resistance of the speaker (currently unwired) is 6.8Ω (nominal is 8Ω)
a.palfreyman
pfm Member
Quick look at the power amp section. The bc212 input transistor is loaded with a 680R (across the VAS transistor base-emitter junctions) which makes the 680R a roughly 1mA current source. If your input bc212 has a gain of say 200, then the base current is 1mA/200 or 5uA out of the base. 5uA times by 18k base resistor value is roughly +0.1V DC voltage at this base.
With two diodes in the bias string and three Vbe junctions (2 drivers and 1 output device) the output pair are ONLY JUST switched on. So +/-0.1V offset doesn't seem unreasonable.
Rebuild and enjoy some power chords.
With two diodes in the bias string and three Vbe junctions (2 drivers and 1 output device) the output pair are ONLY JUST switched on. So +/-0.1V offset doesn't seem unreasonable.
Rebuild and enjoy some power chords.
It does seem that despite the use of excellent quality components throughout the build, some of the electronic design is a little "less than optimal".
This would fit with Badger being a startup/short lived company; around 2-3 years as far as I can find.
I also discovered (I felt a tingle...) that the heatsinks on the 2 output transistors are not earthed, or indeed isolated - per the BD711 datasheet, the collector is connected to the body, and the body is connected to the heatsink...
This would fit with Badger being a startup/short lived company; around 2-3 years as far as I can find.
I also discovered (I felt a tingle...) that the heatsinks on the 2 output transistors are not earthed, or indeed isolated - per the BD711 datasheet, the collector is connected to the body, and the body is connected to the heatsink...