Moved from another thread....
I have this dac (Bricasti M1) and I like it very much, but to my ears the treble (slightly) lacks ultimate resolution and sweetness compared with my other dacs. On bright and older recordings it can tend to be a little "unforgiving".
Taking a look at the pcbs's I noticed AD843 op-amps in the IV and buffer stages. These op-amps are not noted for audiophile qualities but they are advertised for high speed applications including 'high speed integration'.
In an effort to add some sweetness I decided to bias all of the op-amps into class A. I did this by adding a CRD with a series resistor from the op-amp output to the -ve rail. (4 per channel) I mounted them on the underside of the pcb so as not to deface the aesthetics. The resistor was used to set the v drop across the CRD to the mid point of the linear region, which for the specific 2mA CRD used was 8.5 vdc.
Because the op-amps are all paired from the diff outputs from the dac, I decided to batch test and match the CRD's to <1%, which was possible because I had a large stock on hand. I also found that the CRD's ranged in value from 1.7 to 2.3mA, but nominally 1.9mA.
I also tested the current sink regulation over a 3V range and it was a creditable 99.8%.
While I had the dac apart I also passed a cursory eye over the linear power supplies which are all discrete types using op-amps to generate references and with tip120/125 darlington output transistors.
I noticed that a 104 film cap had been installed across the bridge rectifier input (transformer secondaries) and another at the bridge output, but there was no series resistor with the input 104, so it obviously isn't an effective "snubber". This seemed odd, so I subsequently unsoldered one side and added a 330r resistor in series ( 2 per channel), though I didn't use the cro to check, from past experience those values are usually effective at damping any ringing.
OK, so how did it sound after this change you might ask?
Actually, I was very surprised to hear such a big change, because frankly, I wasn't able to measure anything substantial on the pc based analyzer. (distortion was already a low 0.001%)
However, the treble is now MUCH sweeter and more pristine than I could have ever imagined possible, and all of those bad old recordings are now very easy on the ear. It's now every bit the equal, and then some, of my Berkeley Alpha 2 in the treble and is easily the best overall and least 'digital' sounding of all my processors. I also now prefer to use the linear filter set rather than the minimum phase set.
It's worth noting though that I made two changes at the same time, so it's difficult to ascribe the contribution from each, but I'm assuming it's the 2mA of bias on all op-amps that's been the major factor.
I can't say that this same modification will be as effective for all op-amps, and / or in other implementations, but it's nonetheless worth experimenting with based on my personal experience.
I have this dac (Bricasti M1) and I like it very much, but to my ears the treble (slightly) lacks ultimate resolution and sweetness compared with my other dacs. On bright and older recordings it can tend to be a little "unforgiving".
Taking a look at the pcbs's I noticed AD843 op-amps in the IV and buffer stages. These op-amps are not noted for audiophile qualities but they are advertised for high speed applications including 'high speed integration'.
In an effort to add some sweetness I decided to bias all of the op-amps into class A. I did this by adding a CRD with a series resistor from the op-amp output to the -ve rail. (4 per channel) I mounted them on the underside of the pcb so as not to deface the aesthetics. The resistor was used to set the v drop across the CRD to the mid point of the linear region, which for the specific 2mA CRD used was 8.5 vdc.
Because the op-amps are all paired from the diff outputs from the dac, I decided to batch test and match the CRD's to <1%, which was possible because I had a large stock on hand. I also found that the CRD's ranged in value from 1.7 to 2.3mA, but nominally 1.9mA.
I also tested the current sink regulation over a 3V range and it was a creditable 99.8%.
While I had the dac apart I also passed a cursory eye over the linear power supplies which are all discrete types using op-amps to generate references and with tip120/125 darlington output transistors.
I noticed that a 104 film cap had been installed across the bridge rectifier input (transformer secondaries) and another at the bridge output, but there was no series resistor with the input 104, so it obviously isn't an effective "snubber". This seemed odd, so I subsequently unsoldered one side and added a 330r resistor in series ( 2 per channel), though I didn't use the cro to check, from past experience those values are usually effective at damping any ringing.
OK, so how did it sound after this change you might ask?
Actually, I was very surprised to hear such a big change, because frankly, I wasn't able to measure anything substantial on the pc based analyzer. (distortion was already a low 0.001%)
However, the treble is now MUCH sweeter and more pristine than I could have ever imagined possible, and all of those bad old recordings are now very easy on the ear. It's now every bit the equal, and then some, of my Berkeley Alpha 2 in the treble and is easily the best overall and least 'digital' sounding of all my processors. I also now prefer to use the linear filter set rather than the minimum phase set.
It's worth noting though that I made two changes at the same time, so it's difficult to ascribe the contribution from each, but I'm assuming it's the 2mA of bias on all op-amps that's been the major factor.
I can't say that this same modification will be as effective for all op-amps, and / or in other implementations, but it's nonetheless worth experimenting with based on my personal experience.
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