It adjusts the max allowed current and if too low, bias current on HPA wont be enough. One way to test is you need at least 300mA at 24v to give some margin. Connect a 10W 80ohm (or close) load resistor to DC step up. Adjust until it can maintain 24v. That’s 7w burning off so be careful it’s hot. Or adjust using HPA as load - increase until you get full bias current on HPA (wherever that may be). Circa 80mA to 120mA.
Testing using the amp as the load is that the current going to Q4 that I need to probe? So now the basic question. Do I connect the probe to the leg going to ground and the leg connected to R15?
Measuring across R14 I come up with 100mA and it is insensitive to adjustment so perhaps the DC stepup board doesn't adjust below minimum required?
The adjustment on the DC-DC step up is probably very coarse and may have quite a high minimum. As long as you are able to get your 100mA per channel that’s fine. Also, the current adjustment on the DC-DC probably varies current by adjusting voltage. But you are measuring current across a CCS, which is designed to maintain a constant current despite changes in voltage.
Well bench tests went OK using a phone as the source and earbuds for testing. Thinking I was good to go I hooked it into my system and after about 15 seconds of playing I started to get massive distortion through the speakers, tried headphones with the same result. The distortion is independent of volume or source selection and happens even when I unplug all sources. For some reason I thought of the heatsinks on the DC step up as something to check. They are very close to the side of the chassis. If they are touching or very close could this cause distortion? What other things should I check?
Attachments
Hi Spiggs,
You have a very nice build there - great craftsmanship and care to placement. What concerns me is that it took 15 seconds for the distortion to show up. Sort of indicates something burned out or went bad. When you say distortion, do you mean garbled sound or or scratchy sound, or weak sound?
You say that it worked fine on earbuds, why did you connect a headphone amp to speakers - and how many ohms impedance are the speakers? This amp should not be used to drive anything less than 30ohms.
Things to check
1. What is your Vcc after the cap mx and CRC - voltage at the Vcc input pin on the daughtercard
2. What is the bias current across the source resistor (R15) of the CCS DN2540? It needs to be circa 100mA.
3. What is voltage (relative to GND) of the bottom of R15? It should be about 1/2 of Vcc.
43. Does it still play your earbuds ok?
5. What is the bias current through the LTP (R4A or R4B)?
You have a very nice build there - great craftsmanship and care to placement. What concerns me is that it took 15 seconds for the distortion to show up. Sort of indicates something burned out or went bad. When you say distortion, do you mean garbled sound or or scratchy sound, or weak sound?
You say that it worked fine on earbuds, why did you connect a headphone amp to speakers - and how many ohms impedance are the speakers? This amp should not be used to drive anything less than 30ohms.
Things to check
1. What is your Vcc after the cap mx and CRC - voltage at the Vcc input pin on the daughtercard
2. What is the bias current across the source resistor (R15) of the CCS DN2540? It needs to be circa 100mA.
3. What is voltage (relative to GND) of the bottom of R15? It should be about 1/2 of Vcc.
43. Does it still play your earbuds ok?
5. What is the bias current through the LTP (R4A or R4B)?
Last edited:
Full setup info. Schiit Modi and Mani as source into the pre, pre outs to ACA and subwoofer, speakers Fostex FF105WK 8 ohm. I get the distortion with headphones plugged in (Meze 99 Classics) or through the ACA into speakers. Distortion is like loud scratching sounds. I tried it 3 times, power off, power on, about 15 seconds then distortion each time. Pulled it out and back on the bench with ear buds plugged in and top off. Heard perhaps a little distortion at first but then it played cleanly for 30 minutes.
Some observations, if I switch to an unconnected source and turn the volume all the way up I can hear the music very low, other than that background is silent.
Poking around doesn't reveal anything, jiggling the internal wires from input to selector around sometimes I got a faint buzz when touching them.
I checked bias current across R14 and it was 100mA, I'll try and check the things you list below and report back.
Some observations, if I switch to an unconnected source and turn the volume all the way up I can hear the music very low, other than that background is silent.
Poking around doesn't reveal anything, jiggling the internal wires from input to selector around sometimes I got a faint buzz when touching them.
I checked bias current across R14 and it was 100mA, I'll try and check the things you list below and report back.
Not that it nessesary is so for this case but if you have other units that Schiit try hook it up for a few minuttes and see what happens, its because have seen quite e few objective cases where schiit units hooked up to professional analyzers make quite some none expected noice and interference.
Hi Spiggs,
You have a very nice build there - great craftsmanship and care to placement. What concerns me is that it took 15 seconds for the distortion to show up. Sort of indicates something burned out or went bad. When you say distortion, do you mean garbled sound or or scratchy sound, or weak sound?
You say that it worked fine on earbuds, why did you connect a headphone amp to speakers - and how many ohms impedance are the speakers? This amp should not be used to drive anything less than 30ohms.
Things to check
1. What is your Vcc after the cap mx and CRC - voltage at the Vcc input pin on the daughtercard
2. What is the bias current across the source resistor (R15) of the CCS DN2540? It needs to be circa 100mA.
3. What is voltage (relative to GND) of the bottom of R15? It should be about 1/2 of Vcc.
43. Does it still play your earbuds ok?
5. What is the bias current through the LTP (R4A or R4B)?
1. 19.66v at the pin closest to the x1 label on the DB
2. 102mA
3. 11.17v
4. playing on bench with ear buds now seems ok
5. 4a and 4b both about 1.07 mA
Don't think it is the sources, these have been part of the system for some time and in fact I usually play through a Schiit Vali into the ACA and it sounds clean. Also I get the distortion even with the sources disconnected.
Check your DC-DC step up output voltage. Is it stable? The fact that things happen repeatedly after 15 seconds may be related to the voltage supply. 15 seconds is how long the cap Mx takes to come up to full voltage. You may have a bad electrolytic capacitor that fails at max voltage of 48v. Measure voltage at the daughterboard as you turn on and wait for the scratchy sound to appear.
So I hooked up another Modi I have and my Meze headphones with it still on the bench and let it play for 30 min and it sounded perfect. Voltage remained stable. I have one test track that is just silence and I would get some noise when turning the volume pot but silence after even when set at full volume. I am just not able to duplicate the issue I had earlier when it was plugged in to the ACA in place of the Vali. I guess I will try it again and see if I can isolate some part of the hookup that causes distortion.
I did want to say thank you for the kind words. Being new to this I am glad it looks quality.
Just spent some more time listening to it on the bench and am really liking the sound through my Meze headphones. The gain is a better match than that of the Vali with usable volume and the sound it's pushing through is fantastic. A real step above. Now I just need to figure out why it was not happy fully integrated into my system.
Hi Spiggs,
Thanks for talking the measurements. They look nominal. Fine.
So the problem is when driving ACA? Skylar88 drives ACA just fine no issues. Is your ACA built to published plans or is it modified? What input coupling cap is on ACA?
It might be an oscillation issue but that would happen instantly upon connection. Try adding a 600 to 1k ohm resistor in series between the preamp and ACA and see if that helps.
Thanks for talking the measurements. They look nominal. Fine.
So the problem is when driving ACA? Skylar88 drives ACA just fine no issues. Is your ACA built to published plans or is it modified? What input coupling cap is on ACA?
It might be an oscillation issue but that would happen instantly upon connection. Try adding a 600 to 1k ohm resistor in series between the preamp and ACA and see if that helps.
ACA is built to published spec using the boards from the diyaudio store. input caps are
https://www.digikey.com/product-detail/en/cornell-dubilier-electronics-cde/381LX332M035H012/338-3789-ND/1700500
The one additional item is on the RCA input I added some smaller caps to act as a first order filter at 130Hz. 2 per channel for .136UF.
https://www.digikey.com/product-detail/en/wima/MKP4G026802F00KSSD/1928-1552-ND/9370481
However I just did one more listening session on the bench with just headphones and did hear the distortion. This time after a few minutes then it went away after a few minutes more and did not come back after listening for another 20 minutes at which point I shut it off. I don't think as loud this time but same sound. Tested voltage while it was happening and it was stable.
So previously I heard it when driving the ACA or headphone in the living room system and now for the first time I heard it while on the bench in the garage driving just the headphones.
https://www.digikey.com/product-detail/en/cornell-dubilier-electronics-cde/381LX332M035H012/338-3789-ND/1700500
The one additional item is on the RCA input I added some smaller caps to act as a first order filter at 130Hz. 2 per channel for .136UF.
https://www.digikey.com/product-detail/en/wima/MKP4G026802F00KSSD/1928-1552-ND/9370481
However I just did one more listening session on the bench with just headphones and did hear the distortion. This time after a few minutes then it went away after a few minutes more and did not come back after listening for another 20 minutes at which point I shut it off. I don't think as loud this time but same sound. Tested voltage while it was happening and it was stable.
So previously I heard it when driving the ACA or headphone in the living room system and now for the first time I heard it while on the bench in the garage driving just the headphones.
The CDE 381LX332M035H012 3300uF 35v cap looks like the output coupling cap C1, not input coupling cap. That's C3, usually a 10uF. Typical choice would be an Elna Silmic II 10uF 35v (a tiny one).
Can you show how you connected the 0.068uF Wima film cap? What do you mean by a first order filter - in conjunction with which resistor? If you put it after the 10k input and tied one end to gnd, that would be a 234Hz low pass to ground, and the music would sound like it is playing through a subwoofer. Maybe you can sketch what you did to the schematic above.
Can you show how you connected the 0.068uF Wima film cap? What do you mean by a first order filter - in conjunction with which resistor? If you put it after the 10k input and tied one end to gnd, that would be a 234Hz low pass to ground, and the music would sound like it is playing through a subwoofer. Maybe you can sketch what you did to the schematic above.
Sorry yes input cap is
https://www.digikey.com/product-detail/en/elna-america/RFS-25V100ME3-5/604-1050-ND/970211
Updated the schematic to show 2 caps in parallel before input to create a value of .136 UF per channel. Goal was a high pass line level filter before the ACA, caps are connected between the RCA input jack and ACA pcb.
https://www.digikey.com/product-detail/en/elna-america/RFS-25V100ME3-5/604-1050-ND/970211
Updated the schematic to show 2 caps in parallel before input to create a value of .136 UF per channel. Goal was a high pass line level filter before the ACA, caps are connected between the RCA input jack and ACA pcb.
Attachments
So you are using the ACA as an active amp in a multiway speaker then with a 117Hz high pass first order XO? I don't see anything that can be causing distortion other than maybe there is overheating going on with your TO220 parts on the amp daughterboards and they are getting too hot after a certain time. 100mA with about 10v drop each is 1W per TO220 - should be ok for the local heatsink fins you have.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.