I received a reply from the designer and will post it with his permission
Such a behaviour is to be expected from any very high gain, DC coupled, servo controlled, amplifier. Remember that this preamp has a LF gain of 80dB (10,000)! I think I mentioned about this behaviour somewhere on my web site. These transients are the price you pay for "no caps in the signal path" coupled with 60dB @ 1KHz gain. You know, you can't have the cake and eat it too
All HPS DC coupled preamps have the same behaviour and all equivalent commercial designs are experiencing the same kind of power up transients.
Such transients are another reason why a rumble filter is recommended. Being a high pass filter, it will filter out any low frequency transients. The alternative is to give up the DC coupling and use caps in the signal path. The caps will actually implement a first order high pass filter, with the same effect in filtering the perturbations, but unfortunately also impacting (clearly audible) the group delay. Another option is to use some sort of delayed signal connection, similar with a power amp speaker connection control, but with a larger time constant.
Myself, even with a rumble filter at the output, I never disconnect the RIAA preamps from the mains.
The only thing to check is the current through the input JFETs. If the devices you got have accidentally a very high Idss, then you may need to decrease a little the common drain resistor, so that the cascode is fairly enough from saturation (at least 3V Vce after the preamp output settles). Keeping the cascode to close to saturation may exacerbate the power on transients.
Otherwise, if the output settles to zero +/-10mV (before the output cap) you got it right. Congratulations, happy listening, and let me know how you find it!
Such a behaviour is to be expected from any very high gain, DC coupled, servo controlled, amplifier. Remember that this preamp has a LF gain of 80dB (10,000)! I think I mentioned about this behaviour somewhere on my web site. These transients are the price you pay for "no caps in the signal path" coupled with 60dB @ 1KHz gain. You know, you can't have the cake and eat it too
All HPS DC coupled preamps have the same behaviour and all equivalent commercial designs are experiencing the same kind of power up transients.
Such transients are another reason why a rumble filter is recommended. Being a high pass filter, it will filter out any low frequency transients. The alternative is to give up the DC coupling and use caps in the signal path. The caps will actually implement a first order high pass filter, with the same effect in filtering the perturbations, but unfortunately also impacting (clearly audible) the group delay. Another option is to use some sort of delayed signal connection, similar with a power amp speaker connection control, but with a larger time constant.
Myself, even with a rumble filter at the output, I never disconnect the RIAA preamps from the mains.
The only thing to check is the current through the input JFETs. If the devices you got have accidentally a very high Idss, then you may need to decrease a little the common drain resistor, so that the cascode is fairly enough from saturation (at least 3V Vce after the preamp output settles). Keeping the cascode to close to saturation may exacerbate the power on transients.
Otherwise, if the output settles to zero +/-10mV (before the output cap) you got it right. Congratulations, happy listening, and let me know how you find it!
I've just finished building the HPS 5.1, it is now playing in the background.
It was a pain to get the thing going, the first time i powered it up i got the 158R resistors to smoke and after that i even managed to broadcast in RF with it, shortly killing the radio and wireless LAN. It is now working fine, although the treble response is a bit poor, this must mean that i need to replace the needle or cartridge ( it is 20 years old ) and i get quite a bit of hum, since it is mounted on a wooden plank instead of a metal box.
It was a pain to get the thing going, the first time i powered it up i got the 158R resistors to smoke and after that i even managed to broadcast in RF with it, shortly killing the radio and wireless LAN. It is now working fine, although the treble response is a bit poor, this must mean that i need to replace the needle or cartridge ( it is 20 years old ) and i get quite a bit of hum, since it is mounted on a wooden plank instead of a metal box.
How many of you that have completed the build and have less than 10mV p2p before the output capacitor ?
I am afraid that my board is oscillating, because after listening to the thing for a few hours yesterday it was so hot that i couldn't keep my hand on it, and today i see that the transients do not settle down anymore, they are somewhere between 1volt p2p before the output cap. Similar experiences anyone ?
I will try to increase the Jfet common resistor to see what happens.
I am afraid that my board is oscillating, because after listening to the thing for a few hours yesterday it was so hot that i couldn't keep my hand on it, and today i see that the transients do not settle down anymore, they are somewhere between 1volt p2p before the output cap. Similar experiences anyone ?
I will try to increase the Jfet common resistor to see what happens.
Actually, you can use the radio shack prototyper boards to fit the SOT transistors: I think this was an MMB5087 that I had on the workbench:
Just slice the gate connection with an Exacto knife.
An externally hosted image should be here but it was not working when we last tested it.
Just slice the gate connection with an Exacto knife.
Firstly, a big thanks to syn08 for the work he has not only done but shared with us, both here and on his web site.
Having recently built a simple phono amp (compared to what I see here anyway) I am on a rapid learning curve wrt low noise, RIAA ect ect! And I probably have many questions but I’ll leave that for another day.
The main reason for the post was to ask if anyone has successfully built the HPS 5.1 amp and how good is it sound wise? I have access to a prototyping milling machine so cutting a PCB should not be an issue (the Gerber files are accepted by the machines software) so was thinking of building the circuit. So if anyone has come across any issues when constructing the phono amp I would obviously be interested
Thanks for any info,
Karl
Having recently built a simple phono amp (compared to what I see here anyway) I am on a rapid learning curve wrt low noise, RIAA ect ect! And I probably have many questions but I’ll leave that for another day.
The main reason for the post was to ask if anyone has successfully built the HPS 5.1 amp and how good is it sound wise? I have access to a prototyping milling machine so cutting a PCB should not be an issue (the Gerber files are accepted by the machines software) so was thinking of building the circuit. So if anyone has come across any issues when constructing the phono amp I would obviously be interested
Thanks for any info,
Karl
@fasterbyelan
I managed to solve the issues i listed above, you can search the thread for other problems encountered by fellow builders.
Soundwise the HPS5.1 is exceptional.
I would suggest you order 2mm thick PCB's as they will get heavy and bend.
After you solder the power connector onto the PCB pour glue over it otherwise you might damage the thin PCB tracks when pulling on the connector.
Use Panasonic FM capacitors. The board generates quite a lot of heat and i imagine regular caps won't have a long lifespan.
Order trafos with 750mA secondary current rating.
I managed to solve the issues i listed above, you can search the thread for other problems encountered by fellow builders.
Soundwise the HPS5.1 is exceptional.
I would suggest you order 2mm thick PCB's as they will get heavy and bend.
After you solder the power connector onto the PCB pour glue over it otherwise you might damage the thin PCB tracks when pulling on the connector.
Use Panasonic FM capacitors. The board generates quite a lot of heat and i imagine regular caps won't have a long lifespan.
Order trafos with 750mA secondary current rating.
Did you raise a BOM? I was going to start one but notice you had one from a an early post. Would be grateful to recieve such.
Karl