One way of removing the input DC coupling cap on the Tripath chip amps is via transformer. Remember in these Tripath chips the input is biases to 2.5V & the cap is as much to keep this away from the signal line coming in as it is to also block any DC that might already be on the line coming in.
Transformers work in this configuration because one end of the secondary winding is connected to the Tripath pin on which the 2.5V bias is generated (in the case of the TA2020 chip this is pin 14 - called the biascap) & the other end of the secondary winding goes directly to the capless input of the tripath.
So in my case I have a transformer on the output of the DAC and the secondaries connected straight to the Tripath as above - a DC coupling. Removing the cap noticeably removes the smearing in HF & lower LF along with the fact that there is now no turn on pop of any sort even though there is 10mV & 55mV on the speakers out (I'll be removing this shortly with a DC offset circuit.
Diagrams attached
Transformers work in this configuration because one end of the secondary winding is connected to the Tripath pin on which the 2.5V bias is generated (in the case of the TA2020 chip this is pin 14 - called the biascap) & the other end of the secondary winding goes directly to the capless input of the tripath.
So in my case I have a transformer on the output of the DAC and the secondaries connected straight to the Tripath as above - a DC coupling. Removing the cap noticeably removes the smearing in HF & lower LF along with the fact that there is now no turn on pop of any sort even though there is 10mV & 55mV on the speakers out (I'll be removing this shortly with a DC offset circuit.
Diagrams attached
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Now some questions:
The biggest problem I have with all this is grounding.
- I have it running from a USB DAC from a Laptop
- because the Tripath amp now has a signal ground at 2.5V I can't ground this.
- I have a little bit of hiss which I'm trying to tie down - when I connect the power ground of the DAC to the power ground of the amp it lessens it to very nearly quiet. It also doesn't change with volume & it's not there when the DAC is disconnected so I'm thinking it's either ground issues or HF hash coming from the output of the PCM1793 DAC.
I'll put an LPF filter on the output of the DAC & test this - something severe like a 30KHz cut-off, just to hear if that's it
The biggest problem I have with all this is grounding.
- I have it running from a USB DAC from a Laptop
- because the Tripath amp now has a signal ground at 2.5V I can't ground this.
- I have a little bit of hiss which I'm trying to tie down - when I connect the power ground of the DAC to the power ground of the amp it lessens it to very nearly quiet. It also doesn't change with volume & it's not there when the DAC is disconnected so I'm thinking it's either ground issues or HF hash coming from the output of the PCM1793 DAC.
I'll put an LPF filter on the output of the DAC & test this - something severe like a 30KHz cut-off, just to hear if that's it
I put an LPF on one channel of the DAC output consisting of a 4.7K resistor in series on each differential signal lead from the DAC (L+ & L-) and a 100nF cap to ground from each lead, which gives approx a cut-off of 33KHz
No change in the hiss so it's probably not HF hash from the DAC if my calcs are correct & the LPF is low enough?
No change in the hiss so it's probably not HF hash from the DAC if my calcs are correct & the LPF is low enough?
Hi jkeny. I'm following your DAC+tranny to t-amp with great interest, but I have doubts of whether I'll be of much help.
About your noise, I too have hiss on my cheap sure amps, but not on my 41hz amps and I'm not sure why yet. I removed some of the hiss when I changed the power caps, but some still remain.
Maybe it's from the unshielded coils?
I can't remember what youre powering with, but if it's a SMPS then what is the switching frequency? Some PSUs switch too slowly and makes audible noise in the audio band.
From you dac tranny thread there was some talk about grounding and I would just like to remind you that signal ground and ordinary ground is not the same on T-amps. Signal input ground must be isolated from real ground (chassis). I'm sry if I'm stating the obvious, as I said I have limited knowledge compared to many here, but compensate in amount of enthusiasm 😀.
About your noise, I too have hiss on my cheap sure amps, but not on my 41hz amps and I'm not sure why yet. I removed some of the hiss when I changed the power caps, but some still remain.
Maybe it's from the unshielded coils?
I can't remember what youre powering with, but if it's a SMPS then what is the switching frequency? Some PSUs switch too slowly and makes audible noise in the audio band.
From you dac tranny thread there was some talk about grounding and I would just like to remind you that signal ground and ordinary ground is not the same on T-amps. Signal input ground must be isolated from real ground (chassis). I'm sry if I'm stating the obvious, as I said I have limited knowledge compared to many here, but compensate in amount of enthusiasm 😀.
Why? Does your Tripath have a biascap pin?
I'll be looking at both of these issues, thanks
This is complicated even more now by floating the signal ground at 2.5V.
Yep, it's confirmed that my noise is coming from the laptop's crappy Dell SMPS. My battery died a while ago & I resisted buying another one as I refuse to spend another 60 euro on one of these. So I couldn't test it using battery power - I ran it off a bench supply. Much cleaner sound & a different noise signature on the scope.
The SMPS is rated at 19.1V at 3.34A but looking at the current usage on the bench supply, it seldom got above 2 amps. Now to find a quiet 19V 2.5A linear supply for this - a Salas Shunt Supply?
The SMPS is rated at 19.1V at 3.34A but looking at the current usage on the bench supply, it seldom got above 2 amps. Now to find a quiet 19V 2.5A linear supply for this - a Salas Shunt Supply?
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nice jkeny. Glad you sorted you noise issue, as that can be really annoying to troubleshoot.
Btw. I haven't quite figured out whether it is necessary to do this bias stuff when using a T-amp or if it just is an experiment but the normal way shown in other threads will work with a t-amp as well as an A/B amp.
Btw. I haven't quite figured out whether it is necessary to do this bias stuff when using a T-amp or if it just is an experiment but the normal way shown in other threads will work with a t-amp as well as an A/B amp.
I just built a Greg Ball series voltage regulator which can do upto 55V at high Amperage. Just need to mount it on heatsink & will test it tonight powering my laptop first & then maybe powering my laptop & Tripath amp. I might even build a Salas Shunt reg to compare against the series Vreg?
I don't know what you mean about the "bias stuff" - it's just done to avoid having a cap on the input. If you consider removing the input cap a good thing then this is the way to go when using a transformer.
I'm sure there's a way of cleverly working in a lightspeed into the input section for volume control, just haven't focused on this yet & others might pitch in here. Unformed thoughts are that the lightspeed is driven by current generated from a 5V supply - could the 2.5V bias be used in any useful way here?
You would then have a very transparent, minimal DAC + amplifier with vol. control 😀
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