Hi TS,
I really hope you do not mind me hijacking your thread - the board will not let me ask a question without using the "search" feature (something I've been doing for hours...).
If a moderator sees fit to annexe my query to a new thread, I'd be very grateful. I really hate to bother the board here, but I don't know of any boards outside of this one to ask about Exicon.
_____________________________________________________________
Over the weekend I have been grappling with an issue regarding the Exicon ECW20N20 / P20 pair.
Although my Atlas analyser (basic for Mosfet testing, I know) reckons they’re fine, I’ve been seeing wild oscillation with bias currents in excess of 250mA and vast distortion below that (oscillation is -31v DC – amp is on +/-50v rails).
I first tried the Exicon EC20/16 pairing in 2001 and have been happy ever since. Seeing as the EC20/16 pair have been replaced by the EC20/20 combo, when it came time to add extra output pairs to my amps, they were the logical option (I wanted more damping factor).
Even at really low voltages on a bench supply, the 4-up / 4-down pairing has given problems (the oscillation frequency is above my 20Meg ‘scope – the 50Meg is in repair, although I reckon it could be above that anyway). As you increase voltage and, particularly bias, all hell breaks loose.
I was concerned the issue could be connected with the drive electronics (LTP / Current Mirror), so I swapped the PCB over to another set of 4-up / 4-down EC20/20s… No oscillation at all, even at bias currents above 2amp (3amps seems to send it oscillating, but I’m not intending to make an amp for the cat to curl around).
The bank of mosfets that’s giving me the grief doesn’t have as well-matched n-channel Gate threshold voltages as the other bank… The maximum disparity is about 0.4mV – the bank that isn’t oscillating matches near-perfectly…
Profusion’s technical guy is on holiday, so I couldn’t get assistance there.
Is this purely down to the disparity in Gate threshold voltages, or is there an inherent stability issue with the N-channel EC20/20? I’ve always been under the impression that the Exicons were inherently stable, until Google revealed this link: http://www.susan-parker.co.uk/zeus-test-2sk1529-fft-1khz.htm
You can’t trust everything you read from Google, so I thought I’d ask here. It took me a long time to cast my eye over the Exicons as I couldn’t believe they’d have an issue… I’ve seen LTP-based op-amps oscillate madly before, so I thought the issue was there and revised the wiring + psu distribution / bypass caps about ten times before I developed a healthy level of cynicism…
When I bought the devices, Profusion assured me they would all be from the same die / production run and that matching wasn’t much of an issue…I had an exhaustive chat with them on this issue, but was told each tube contained devices manufactured in-sequence…
DC offset with the working bank is below 15mV at 1amp bias (+/- 50v)
Is this really just a case of the Gate voltages not being matched, or are we talking about something deeper?
Thanks in advance.
BTW – I’d love to post a schematic of the amp, but it’s copyrighted… It’s a tricked-out version of the Maplin / Hitachi amp, with current sources and several mods. I have 4 of these amps and have been running them since 2001 with no issues at all…until I decided I wanted more damping factor…
Justin
I really hope you do not mind me hijacking your thread - the board will not let me ask a question without using the "search" feature (something I've been doing for hours...).
If a moderator sees fit to annexe my query to a new thread, I'd be very grateful. I really hate to bother the board here, but I don't know of any boards outside of this one to ask about Exicon.
_____________________________________________________________
Over the weekend I have been grappling with an issue regarding the Exicon ECW20N20 / P20 pair.
Although my Atlas analyser (basic for Mosfet testing, I know) reckons they’re fine, I’ve been seeing wild oscillation with bias currents in excess of 250mA and vast distortion below that (oscillation is -31v DC – amp is on +/-50v rails).
I first tried the Exicon EC20/16 pairing in 2001 and have been happy ever since. Seeing as the EC20/16 pair have been replaced by the EC20/20 combo, when it came time to add extra output pairs to my amps, they were the logical option (I wanted more damping factor).
Even at really low voltages on a bench supply, the 4-up / 4-down pairing has given problems (the oscillation frequency is above my 20Meg ‘scope – the 50Meg is in repair, although I reckon it could be above that anyway). As you increase voltage and, particularly bias, all hell breaks loose.
I was concerned the issue could be connected with the drive electronics (LTP / Current Mirror), so I swapped the PCB over to another set of 4-up / 4-down EC20/20s… No oscillation at all, even at bias currents above 2amp (3amps seems to send it oscillating, but I’m not intending to make an amp for the cat to curl around).
The bank of mosfets that’s giving me the grief doesn’t have as well-matched n-channel Gate threshold voltages as the other bank… The maximum disparity is about 0.4mV – the bank that isn’t oscillating matches near-perfectly…
Profusion’s technical guy is on holiday, so I couldn’t get assistance there.
Is this purely down to the disparity in Gate threshold voltages, or is there an inherent stability issue with the N-channel EC20/20? I’ve always been under the impression that the Exicons were inherently stable, until Google revealed this link: http://www.susan-parker.co.uk/zeus-test-2sk1529-fft-1khz.htm
You can’t trust everything you read from Google, so I thought I’d ask here. It took me a long time to cast my eye over the Exicons as I couldn’t believe they’d have an issue… I’ve seen LTP-based op-amps oscillate madly before, so I thought the issue was there and revised the wiring + psu distribution / bypass caps about ten times before I developed a healthy level of cynicism…
When I bought the devices, Profusion assured me they would all be from the same die / production run and that matching wasn’t much of an issue…I had an exhaustive chat with them on this issue, but was told each tube contained devices manufactured in-sequence…
DC offset with the working bank is below 15mV at 1amp bias (+/- 50v)
Is this really just a case of the Gate voltages not being matched, or are we talking about something deeper?
Thanks in advance.
BTW – I’d love to post a schematic of the amp, but it’s copyrighted… It’s a tricked-out version of the Maplin / Hitachi amp, with current sources and several mods. I have 4 of these amps and have been running them since 2001 with no issues at all…until I decided I wanted more damping factor…
Justin
The two posts are years old now, but in case anyone is still interested...
1. TS: The exicon mosfet.zip file posted in: http://www.diyaudio.com/forums/solid-state/4094-lateral-mosfet-replacements-2sk1058-hitachi.html has the following models:
Ecf10n20.txt
Ecf10n25.txt
Ecf10p20.txt
Ecf10p25.txt
Ecf20n20.txt
Ecf20n25.txt
Ecf20p20.txt
Ecf20p25.txt
Ecx10n20.txt
Ecx10p20.txt
2. Justin: I wonder how you got on with your instability problem? The oscillation could be due to a few things; you have just increased the capacitance/lag by heaps, so the possibilities include: need to change zobel network and/or speaker cable; capacitive load on driver is borderline (one set of transistors just ok, the other set not), feedback through power supply, longer lead lengths now you have more devices, and other possibilities that depend on the circuit (e.g. if there is negative feedback around the output stage to the driver; and it is a little-known fact that common-drain stages themselves can have instability - as can common-collector circuits - depending on input and output impedances). ... Ah! I now see there is another post by you 11 days later. I should have looked at that first.
Also: Susan Parker's circuit is push-pull common-drain (source-follower), with transformers on input and output; instability would certainly be a risk there, and the devices she compares the lateral MOSFETs to are very different (much higher capacitance), so her results aren't surprising, although I do wonder what "did not give as good results" means in detail.
1. TS: The exicon mosfet.zip file posted in: http://www.diyaudio.com/forums/solid-state/4094-lateral-mosfet-replacements-2sk1058-hitachi.html has the following models:
Ecf10n20.txt
Ecf10n25.txt
Ecf10p20.txt
Ecf10p25.txt
Ecf20n20.txt
Ecf20n25.txt
Ecf20p20.txt
Ecf20p25.txt
Ecx10n20.txt
Ecx10p20.txt
2. Justin: I wonder how you got on with your instability problem? The oscillation could be due to a few things; you have just increased the capacitance/lag by heaps, so the possibilities include: need to change zobel network and/or speaker cable; capacitive load on driver is borderline (one set of transistors just ok, the other set not), feedback through power supply, longer lead lengths now you have more devices, and other possibilities that depend on the circuit (e.g. if there is negative feedback around the output stage to the driver; and it is a little-known fact that common-drain stages themselves can have instability - as can common-collector circuits - depending on input and output impedances). ... Ah! I now see there is another post by you 11 days later. I should have looked at that first.
Also: Susan Parker's circuit is push-pull common-drain (source-follower), with transformers on input and output; instability would certainly be a risk there, and the devices she compares the lateral MOSFETs to are very different (much higher capacitance), so her results aren't surprising, although I do wonder what "did not give as good results" means in detail.
I just put the EC-10P16 and N16 into my DH-120 with zero problems. I am going to change the gate resistors to 220 and 180 hanging them right off the socket pin. They have half the capacitance as the Hitachi. There are several comments in old Audio Amateur issues that help with this. SEMILAB has a good application guide on L-MOSFETS.
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