Member
Joined 2009
Paid Member
Nice one.
I believe the recommended bias is in the range you are operating it at so you're on the right track. I was aiming for 200mA per pair of output devices. Keep us appraised of the sound as it 'breaks in'.
The VSSA has similar performance specs if I remember correctly - both amps have fairly high feedback factors, both are symmetrical etc. I expect there will be a lot of interest in your A/B'ing.
I believe the recommended bias is in the range you are operating it at so you're on the right track. I was aiming for 200mA per pair of output devices. Keep us appraised of the sound as it 'breaks in'.
The VSSA has similar performance specs if I remember correctly - both amps have fairly high feedback factors, both are symmetrical etc. I expect there will be a lot of interest in your A/B'ing.
Member
Joined 2009
Paid Member
There are some options but measuring the draw on the power rail is perfectly fine - the amount of current flowing in the circuit outside of the power devices is tiny compared with the power devices so when you measure the current through the power rails you are effectively measuring the current through the FETs.
What Greg suggests is taking out the fuses. Replace the fuses with 10R power resistors and measure the voltage drop across each resistor with no load on the amplifier. The current that flows through the FETs must flow through these temporary 10R resistors and cause a voltage drop across them of voltage = current * resistance. So for 200mA x 10R = 0.2V voltage.
You do have a source resistor on the FETs of 0.15R so you could also measure the voltage across those resistors. But they are smaller resistors so the voltage drop is smaller and therefore it can be less accurate to measure. It also depends on the accuracy of these resistors - are they exactly 0.15R. The larger 10R resistors might be more accurate. I don't have a preference as to which method is best.
Be careful about probing around though as there's a risk your multi-meter probe could slip and cause a short somewhere - I've destroyed parts like that !
Anyhow, this is not like an emitter follower Class AB amplifier where you want to fine tune the bias to a particular value. Instead, I believe the SKA has a lot of tolerance and you can bias it anywhere you please from about 50mA upwards - limited only by the size of your heatsink. If it sounds good at lower bias then use it since it wastes less heat.
Do tell us more about the differences in sound between the SKA and the VSSA.
By the way, I also read on Greg's forum that some people heard subtle differences in sound depending on capacitor choice for the feedback shunt capacitor C9.
What Greg suggests is taking out the fuses. Replace the fuses with 10R power resistors and measure the voltage drop across each resistor with no load on the amplifier. The current that flows through the FETs must flow through these temporary 10R resistors and cause a voltage drop across them of voltage = current * resistance. So for 200mA x 10R = 0.2V voltage.
You do have a source resistor on the FETs of 0.15R so you could also measure the voltage across those resistors. But they are smaller resistors so the voltage drop is smaller and therefore it can be less accurate to measure. It also depends on the accuracy of these resistors - are they exactly 0.15R. The larger 10R resistors might be more accurate. I don't have a preference as to which method is best.
Be careful about probing around though as there's a risk your multi-meter probe could slip and cause a short somewhere - I've destroyed parts like that !
Anyhow, this is not like an emitter follower Class AB amplifier where you want to fine tune the bias to a particular value. Instead, I believe the SKA has a lot of tolerance and you can bias it anywhere you please from about 50mA upwards - limited only by the size of your heatsink. If it sounds good at lower bias then use it since it wastes less heat.
Do tell us more about the differences in sound between the SKA and the VSSA.
By the way, I also read on Greg's forum that some people heard subtle differences in sound depending on capacitor choice for the feedback shunt capacitor C9.
Last edited:
Hey Gareth,
Thanks. I have it set at 240mA which seems too high. I will back it off to 200mA in the morning, maybe even try it a little lower. I have some class A amps if I want to heat the room.
I have to think reading the draw through an ammeter is more accurate than across a 10R resistor.
On my amp, C9 is a 7
47uf electro and it goes to ground. The amp sounds good like it is so I think I'll leave things alone at least till it has a chance to burn in. I got a chance to listen to it some more and I really like it. The VSSA is just slightly more detailed but that is not always a good thing depending on the recording. For instance, I played a Jewel album and she likes a lot of "air" on her mic and on the VSSA it is almost too much sizzle but the SKA tames it just enough to really smooth it out. Gerry Mulligan however is superb on the VSSA. I'm going to do a lot of listening to both of them tomorrow and I'll try a few more genres. Warren Hayes sound great on both amps. I think the SKA will be my favorite for female focalists. It i really smooth and full sounding.
For $18.50 delivered, it is a no-brainer to buy these boards. Actually, the partial kit which included the matched FET's and LTP devices is probably even a better deal.
Anyway, highly recommended from my end.
Blessings, Terry
Thanks. I have it set at 240mA which seems too high. I will back it off to 200mA in the morning, maybe even try it a little lower. I have some class A amps if I want to heat the room.
I have to think reading the draw through an ammeter is more accurate than across a 10R resistor.On my amp, C9 is a 7
47uf electro and it goes to ground. The amp sounds good like it is so I think I'll leave things alone at least till it has a chance to burn in. I got a chance to listen to it some more and I really like it. The VSSA is just slightly more detailed but that is not always a good thing depending on the recording. For instance, I played a Jewel album and she likes a lot of "air" on her mic and on the VSSA it is almost too much sizzle but the SKA tames it just enough to really smooth it out. Gerry Mulligan however is superb on the VSSA. I'm going to do a lot of listening to both of them tomorrow and I'll try a few more genres. Warren Hayes sound great on both amps. I think the SKA will be my favorite for female focalists. It i really smooth and full sounding.
For $18.50 delivered, it is a no-brainer to buy these boards. Actually, the partial kit which included the matched FET's and LTP devices is probably even a better deal.
Anyway, highly recommended from my end.
Blessings, Terry
That would be 2V measured.
Greg suggests running it at 100mA with 1V drop accross the 10R test resistor to start with.
Member
Joined 2009
Paid Member
I've studied some of the reports of listening impressions of the SKA and they match my brief listening to my own version of it. What caught my attention and that of others also matches your impressions Terry - the treble is that seemingly very rare combination of being clean-accurate-detailed but also no sizzle, it's amazingly smooth. I have a sensitivity to sizzle that gives me listening fatigue all too easily but I sense the SKA will be one amplifier that really works for me. I must get it finished.
p.s. thanks Repeet for fixing my bad late night math!
p.s. thanks Repeet for fixing my bad late night math!
I'm not quite sure. It is the DX supply. I got it along with the DX Super A boards. Carlos called it a Cap multiplier but also calls it a regulated supply. It doesn't break sweat driving both these boards but I don't think it is quite as clean as the Mr Evil/PMI cap multiplier that I am using with the VSSA. Although it might have something to do with the currents the SKA is pulling. I just know that when I measure the output of the two for AC, the DX shows some AC fluctuation while the music is playing and the PMI CM stays at almost 0vAC. Of course, I have the SKA running on +/-50v and the VSSA is only running at +/-37V. The Mr E/PMI CM will not go above about about 42V. I'm using an E-core 40-0-40vAC transformer and just turn the DX supply down to +/-50V rails.
I got a chance to do a lot of listening to the SKA today. I A/B'd it against some more of my amps. The most comparable amp I have to it is the KSA 50 clone. They are very close in tonal quality. Both have a very smooth high end. The SKA may be just a tad tighter but they are very , very close. Hope this gives some folks an idea what to expect.
Blessings, Terry
Blessings, Terry
Yes, I'm using just the one PSU for both boards. It holds rock solid. I just went out and tested it, and at idle. It is at +/-50.3vDC and at a very loud level it only drops to +/-50.2. That regulator is doing it's job.
While I was out there just now, I hooked up the HB again so I would have fresh ears. This is what I hear. The HB has a little bigger sound-stage. The best I can describe the SKA from the time I have had to listen to it is a "compressed sound". It's not profound but there. All the information seems to be there but the edge is just a little softer. It is very pleasant to listen to.
One thing I learned from all this comparing..... I have some really good sounding amps.
Member
Joined 2009
Paid Member
Hi Terry,
Keep the reports coming !
I have this feeling that the SKA might turn out to live up to it's name - a killer amp. But I don't know that it has been 'voiced'. By that I mean it hasn't been tweaked to achieve a certain 'sound'. It has been designed with a high feedback factor and has low distortion. The OLG is maintained right up past 20KHz so the treble is clean. Being symmetrical it is dominated by 3rd harmonics except at low powers where the 3rd drops below the 2nd. This could result in a more compressed sound at higher volumes - this is essentially what 3rd harmonic distortion is from my limited understanding. However, I've not read anybody reporting that of this amp so I'm following your words with interest. fyi - the 3rd harmonic distortion in the SKA can be pushed as low as you want almost, by increasing the bias. I suspect it's cleaner than the VSSA. The VSSA distortion is more sensitive to any imbalance in the dual feedback networks and power supply noise.
I am used to a clean sounding symmetrical amplifier - my Bryston. So for me the SKA does not sound compressed. But my single ended amps do have more 'air' and I believe it is because they have more distortion, not just 2nd harmonic but more inter-modulation products arising from these 2nd harmonics which add some bling to the music. If you are accustomed to this sound then the cleaner Bryston sound might not be your cup of tea but it is more a case of being used to a little distortion of the 'right kind' than being used to clean output. I would be very interested in a comparison between the SKA and the P3A - almost polar opposites in design. Your Krell clone, being Class A is probably the cleanest amp you have - just a guess on my part though.
But I think it's worth spending more time with the SKA. Are you using an input cap - it may dominate the sound. Others have reported an impact from the choice of the feedback capacitor C9 and the need for the amplifier to burn-in. In terms of 'voicing' I have been considering some options to try on TGM7 - looking at component values and choices.
Keep the reports coming !
I have this feeling that the SKA might turn out to live up to it's name - a killer amp. But I don't know that it has been 'voiced'. By that I mean it hasn't been tweaked to achieve a certain 'sound'. It has been designed with a high feedback factor and has low distortion. The OLG is maintained right up past 20KHz so the treble is clean. Being symmetrical it is dominated by 3rd harmonics except at low powers where the 3rd drops below the 2nd. This could result in a more compressed sound at higher volumes - this is essentially what 3rd harmonic distortion is from my limited understanding. However, I've not read anybody reporting that of this amp so I'm following your words with interest. fyi - the 3rd harmonic distortion in the SKA can be pushed as low as you want almost, by increasing the bias. I suspect it's cleaner than the VSSA. The VSSA distortion is more sensitive to any imbalance in the dual feedback networks and power supply noise.
I am used to a clean sounding symmetrical amplifier - my Bryston. So for me the SKA does not sound compressed. But my single ended amps do have more 'air' and I believe it is because they have more distortion, not just 2nd harmonic but more inter-modulation products arising from these 2nd harmonics which add some bling to the music. If you are accustomed to this sound then the cleaner Bryston sound might not be your cup of tea but it is more a case of being used to a little distortion of the 'right kind' than being used to clean output. I would be very interested in a comparison between the SKA and the P3A - almost polar opposites in design. Your Krell clone, being Class A is probably the cleanest amp you have - just a guess on my part though.
But I think it's worth spending more time with the SKA. Are you using an input cap - it may dominate the sound. Others have reported an impact from the choice of the feedback capacitor C9 and the need for the amplifier to burn-in. In terms of 'voicing' I have been considering some options to try on TGM7 - looking at component values and choices.
Last edited:
The biggest change in sound I found was increasing the on board C from 300u to 1000u.
Changed some of the resistors to .1%.
I used .15 ohms on the source of each FET. Not a single .15 per back of fets.
It locked everything into place; sound stage and bass specifically.
I'm using unregulated supplies; RCRC with schotky diodes and snubbers across them. (See Jim Hagerman's article).
I agree with you that this is a clean sound due to the design methods you've mentioned. I like it. A lot. I'm running 6 of them.
Changed some of the resistors to .1%.
I used .15 ohms on the source of each FET. Not a single .15 per back of fets.
It locked everything into place; sound stage and bass specifically.
I'm using unregulated supplies; RCRC with schotky diodes and snubbers across them. (See Jim Hagerman's article).
I agree with you that this is a clean sound due to the design methods you've mentioned. I like it. A lot. I'm running 6 of them.
Interesting. I have not measured anything. I don't know how to measure for harmonics. I am strictly doing a listening test and only with the speakers I have set up in my room. I have tried a few different combinations of speakers to see if that changes the results. I built a speaker switch using one of these. I use my DCB1 and X-BosoZ to adjust the volumes of the two amps to get them as close as possible to even and then toggle back and forth with the speaker switch. I was actually surprised at how close most of the amps sound to each other once the output is equalized. The words I use to describe what I am hearing may not paint the same picture to some that they do to others, they are simply what makes sense to me. A spectrum analyzer may show better what I am hearing but I no longer have access to one. There are very subtle frequency changes I hear as I switch back and forth. It is not always clear either because music has changes happening all the time during a song that can make you think you heard something change tonally when all that really happened was a change in the song.