TheBrownNote
New Member
Hello, and sorry to not have been monitoring this thread for a while. I thought I'd be notified via email of new posts but I guess it doesn't work that way or maybe I need to adjust my preferences.
FootFungus, 10-4 on your thought that the tests need to be done at operating temps and voltages, I hope to do this. I'm going to test the Hitachi mosfets I have and make sure they work, and if they
do and are matched well enough based on testing I'll keep them. If not I'll have to consider getting new mosfets, but I'm hoping to not have to do that and if I do would probably try to find some vintage Hitachis instead of adding another type into the mix.
As for the differences between the ST-140 and EX-442 schematic, there are clearly some in the schematics available online. I have a ST-140 that (according to the person I bought it from, who seemed honest) was converted to a monoblock by B&K about 8 years ago, though. Based on careful study of all parts in that unit vs. those in the stock stereo version I bought at the same time (also allegedly reconditioned by B&K), the values and placement of all components are identical. The only differences are that the second set of output devices, which remain in the same position as they were in the stereo model, are connected by long twisted pair jumpers all the way over to the other board, the board from the other channel is removed, and the power supply is set up to be one-sided. I'd like to mimic this in my monoblock conversion--I'm not trying to create an EX-442 clone. I want to keep the circuit the same as it is and just put in another power supply and two more output mosfets per channel. I'd be pulling the heat sink out of the monoblock and using both on the back side of the modified unit, by the way, to create adequate cooling, so there's some casework to be done. I can post both schematics if needed, but looking at the EX-442 one isn't directly applicable to my current problem, I think.
Here's a summary of What I've done with the third ST-140 I own, which is the one the extra power supply, heat sink, and four additional output devices from the monoblock will be going into. I replaced the old school "beer can" filter caps with skinny little Mundorf Mlytic AGs (15000uf, and 100v instead of 75v), replaced the electrolytic inputs on the board with inline clunky Mundorf EVO oil caps (47uf, same as original, but 250v instead of 16v), replaced the electrolytic output caps with Jensen film caps (47uf, but 150v instead of 35V), replaced the existing bridge rectifier with an INXS FRED unit (part # VBE17-06NO7), and removed the two electrolytic 100uf 100v rail caps on each board, replacing them with nothing. This last change was on the advice of a guy that did an amazing job completely recreating the topology of my Acurus preamp and power amp; his previous
work sounded so good that I decided to trust him and took the leap of faith to remove these parts. With no more lytic caps in the audio path and better power supply components the result sounds much better than stock. Deeper and wider soundstage, way better clarity and control at all frequencies, no more tizzy highs and less of a boomy, flabby bottom, but still not as full and controlled as the Acurus A-250 (I can provide details about that amp if anybody's interested but it's not directly relevant to this thread). Once I saw the improvement from the initial modifications I wondered what would happen when more power was put into the equation, and this led me to buy the other two ST-140 amps and start this ST-140 Frankenstein project. When I get back in there to do the monoblock conversion I'll also replace all internal wiring in the audio path with and duplicate the power supply changes to the power supply that gets added in, but I don't plan on any other changes. Open to comments always, though. This is basically a simple parts replacement project on the first amp that got out of control and now here I am.
Thanks, Alan, for the sketch of the matching circuit. I'm going to see how far I get with the $30 Chinese part tester and if I don't feel comfortable with the outcome will dig deeper. For now just
waiting weeks for it to arrive, ho hum. Based on your comments and those of Binkman it sounds like the testing process could be quite an odyssey, and if the bargain tester I bought doesn't seem to
have the required accuracy or reliability I'll have to figure out a plan B. Here is the tester, by the way:
https://us.banggood.com/Wholesale-W...stor-Tester-wp-Usa-1083042.html?rmmds=myorder
Thanks for all the feedback!
Jon
FootFungus, 10-4 on your thought that the tests need to be done at operating temps and voltages, I hope to do this. I'm going to test the Hitachi mosfets I have and make sure they work, and if they
do and are matched well enough based on testing I'll keep them. If not I'll have to consider getting new mosfets, but I'm hoping to not have to do that and if I do would probably try to find some vintage Hitachis instead of adding another type into the mix.
As for the differences between the ST-140 and EX-442 schematic, there are clearly some in the schematics available online. I have a ST-140 that (according to the person I bought it from, who seemed honest) was converted to a monoblock by B&K about 8 years ago, though. Based on careful study of all parts in that unit vs. those in the stock stereo version I bought at the same time (also allegedly reconditioned by B&K), the values and placement of all components are identical. The only differences are that the second set of output devices, which remain in the same position as they were in the stereo model, are connected by long twisted pair jumpers all the way over to the other board, the board from the other channel is removed, and the power supply is set up to be one-sided. I'd like to mimic this in my monoblock conversion--I'm not trying to create an EX-442 clone. I want to keep the circuit the same as it is and just put in another power supply and two more output mosfets per channel. I'd be pulling the heat sink out of the monoblock and using both on the back side of the modified unit, by the way, to create adequate cooling, so there's some casework to be done. I can post both schematics if needed, but looking at the EX-442 one isn't directly applicable to my current problem, I think.
Here's a summary of What I've done with the third ST-140 I own, which is the one the extra power supply, heat sink, and four additional output devices from the monoblock will be going into. I replaced the old school "beer can" filter caps with skinny little Mundorf Mlytic AGs (15000uf, and 100v instead of 75v), replaced the electrolytic inputs on the board with inline clunky Mundorf EVO oil caps (47uf, same as original, but 250v instead of 16v), replaced the electrolytic output caps with Jensen film caps (47uf, but 150v instead of 35V), replaced the existing bridge rectifier with an INXS FRED unit (part # VBE17-06NO7), and removed the two electrolytic 100uf 100v rail caps on each board, replacing them with nothing. This last change was on the advice of a guy that did an amazing job completely recreating the topology of my Acurus preamp and power amp; his previous
work sounded so good that I decided to trust him and took the leap of faith to remove these parts. With no more lytic caps in the audio path and better power supply components the result sounds much better than stock. Deeper and wider soundstage, way better clarity and control at all frequencies, no more tizzy highs and less of a boomy, flabby bottom, but still not as full and controlled as the Acurus A-250 (I can provide details about that amp if anybody's interested but it's not directly relevant to this thread). Once I saw the improvement from the initial modifications I wondered what would happen when more power was put into the equation, and this led me to buy the other two ST-140 amps and start this ST-140 Frankenstein project. When I get back in there to do the monoblock conversion I'll also replace all internal wiring in the audio path with and duplicate the power supply changes to the power supply that gets added in, but I don't plan on any other changes. Open to comments always, though. This is basically a simple parts replacement project on the first amp that got out of control and now here I am.
Thanks, Alan, for the sketch of the matching circuit. I'm going to see how far I get with the $30 Chinese part tester and if I don't feel comfortable with the outcome will dig deeper. For now just
waiting weeks for it to arrive, ho hum. Based on your comments and those of Binkman it sounds like the testing process could be quite an odyssey, and if the bargain tester I bought doesn't seem to
have the required accuracy or reliability I'll have to figure out a plan B. Here is the tester, by the way:
https://us.banggood.com/Wholesale-W...stor-Tester-wp-Usa-1083042.html?rmmds=myorder
Thanks for all the feedback!
Jon