Beautiful. Thanks. 🙂
The 0.47R should be 0.22R.
Edit: oh never mind they are doubled up parallel - good.
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the 2k2 mistake gave your amplifier an extra 17dB of feedback. You are lucky that the 5pF for C5 stopped it oscillating.So looking at the schematic shows that the gain is set by R12 over R11 or 22k/1k=22 or about 27dB. Maybe that is the difference I am hearing - 4 dB difference?
Edit: Ok, bonehead mistake on my part. I had a mislabeled bag of 2.2k 1w resistors labeled as 22k. So gain was only 2.2 😛
Just replaced with a pair of 47k 1/4w in parallel for 23.5k 1/2w. Sounds great now 🙂
Nice sounding amp!
Oscillation creates excessive heat, which can damage the amp. But the phenomenon can also damage the tweeters
So far, the FX8 Bimo mod appears to measure the best with my speakers in terms of lowest harmonic distortion (as measured with a microphone). The FX8 Bimo mod seems to have a slight 1.5dB depression in the response though from 2kHz to 5-7kHz as compared to the other amps which all track identically through that region. I wonder what can be causing this?
Measurements made with calibrated UMIK-1 (Cross Spectrum Labs calibrated), GRS Research XL-S speaker, all at 0.5m distance on tweeter axis with speaker on a 30in tall stand for tweeter 42in above floor. All amps used same Dx PSU with 35v rails and 250VA transformer.
Amps tested include: FX8 (5cm x 5cm version), FH9 (FX8 5cm modified with vertical FETs), Dx amp, and Inv JFET Circlophone.
Here is measured frequency response of speaker with different amps (FX8 Bimo is red curve):
Here is measured HD of speaker with FX8 5cm (note rising HD above 1kHz) - all values in legend are with cursor at 5kHz mark:
Here is measured HD of speaker with FH9 - the simple switch to vertical FETs with source/collector resistors and Vbe multiplier fixed the rising distortion problem seen above (same exact board):
Here is measured HD of speaker with Dx amp:
Here is measured HD of speaker with Inv JFET Circlophone amp:
Here is measured HD of speaker with FX8 Bimo mod - and in case you are wondering if something was amiss with the lateral FETs used in the FX8 5cm above that may be causing the rising distortion, the same exact later FETs from that amp were used in this amp:
Note that all the amps sound very good, excellent even. The Circlophone had the edge all along but now I think the FX8 Bimo may be close or better once I figure what is causing that mild -1.5dB depression in the response from 2kHz to 7kHz.
thanks for that data, x. just the trigger i needed to go with fx8. the distortion with the bimo mod looks very similar to dx, which's said to be a warm sounding amp, which's what i needed. good to know that circlophone is in the same family of sound. little more surprising is fh9 because it would be the first cfp amp that i've heard with the similar sound to dx. all other cfp amps i've dealt with had lively yet 'cold' sound.
Update on FX8 Bimo mod. The slight dip from 2k to 7k was indeed caused by lack of output inductor. It essentially had a 4.7ohm padding resistor in the tweeter. With the inductor in place the amp tracks the frequency response like the other amps.
Donovas: you are welcome. Any of these amps sound great and it's quite surprising how well a simple design like Dx which doesn't even have a true CCS sounds and measures well. Of these, the FX8 Bimo seems to have the lowest HD still but if you want cheap easy to find vertical FETs, the FH9 is certainly a nice amp that won't disappoint. The upgrades that Bimo put in that probably make it so much better are choice of LTP transistors, higher voltage VAS transistors, an LED biased CCS, no Zeners or diodes in circuit at output stage and bigger rail caps. I wonder if these upgrades combined with a VBE multiplier and vertical FETs would make an FH9 Bimo edition even better than last FH9?
Donovas: you are welcome. Any of these amps sound great and it's quite surprising how well a simple design like Dx which doesn't even have a true CCS sounds and measures well. Of these, the FX8 Bimo seems to have the lowest HD still but if you want cheap easy to find vertical FETs, the FH9 is certainly a nice amp that won't disappoint. The upgrades that Bimo put in that probably make it so much better are choice of LTP transistors, higher voltage VAS transistors, an LED biased CCS, no Zeners or diodes in circuit at output stage and bigger rail caps. I wonder if these upgrades combined with a VBE multiplier and vertical FETs would make an FH9 Bimo edition even better than last FH9?
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Sonal Kunal - could you please provide Gerber file for FH9, and if possible add marking to say Apex FH9 xrk mod?
Thanks,
X
Dear friends,
When you next post any pcb file, please post the corresponding schematic also (only one more click) such that lot of searching back and forth can be avoided.
Thanks to everybody.
--gannaji
When you next post any pcb file, please post the corresponding schematic also (only one more click) such that lot of searching back and forth can be avoided.
Thanks to everybody.
--gannaji
on which transistors can be replaced by MJL 1194 MJL 1193? and MJE 15032 MJE15033
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FX8 50mm Stereo
I whipped these together quickly as the 50mm FX8 is a very simple build. I have all correct parts this time (22k resistors). I increased the power rail caps to 100uF 63v (from 47uF 50v) and am using low ESR 330uF OSCONs for the feedback reservoir cap. All bypass caps are film caps and the only ceramic is the 10uF 50v MLCC on the inputs (still waiting for the Wima's). The BC546C LTP's are Hfe matched exactly and thermally bonded. Both amps are mounted on a rather compact heat sink that will have a single 12v fan to keep things cool. The SK1058 and SJ162's are from a different supplier, this time from China - we will see how they sound when I fire it up tomorrow. Considering a compact 35v rail SMPS to go with this amp. Heatsink is 150mm x 60mm x 25mm.
I whipped these together quickly as the 50mm FX8 is a very simple build. I have all correct parts this time (22k resistors). I increased the power rail caps to 100uF 63v (from 47uF 50v) and am using low ESR 330uF OSCONs for the feedback reservoir cap. All bypass caps are film caps and the only ceramic is the 10uF 50v MLCC on the inputs (still waiting for the Wima's). The BC546C LTP's are Hfe matched exactly and thermally bonded. Both amps are mounted on a rather compact heat sink that will have a single 12v fan to keep things cool. The SK1058 and SJ162's are from a different supplier, this time from China - we will see how they sound when I fire it up tomorrow. Considering a compact 35v rail SMPS to go with this amp. Heatsink is 150mm x 60mm x 25mm.
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I tried the FX8 with a +-36V Connex SMPS. Not a good match. This amp has poor PSRR so it really needs a clean PSU. I can actually hear the SMPS in the speakers. I hope you have better luck with yours.
I have not built that many amps. Out of all the Apex amps I have built - the FX8 Bimo and FH9 are about tied. I am hoping the FX8 50mm here cleans up in the HD department now that I am using different components.
Of all the amps I have tried in Solid State Forum, the inverted JFET Circlophone is still my favorite. I am told it does not need a clean power supply and can sound good still. But you know what, the Dx amp sounds quite nice and for such a simple design that is $18 shipping included for a pair, parts and PCB - you really can't go wrong.
I am surprised the PSRR is not good - if I look at the SK/SJ spec sheets, the drain current vs drain-source voltage, for gate voltage circa 3v is very flat - which I am told helps with achieving a higher PSRR as variations in rail voltage don't translate to a large variation in drain current.
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