I've been modding my 508ESD with Burr Brown PCM58P DACS and have been following the discussions on this board about OPA627, which I selected for the op amps.
The concern I have is that the OP AMP supply on the 508 is +/-5V, which is less than most other applications I've read.
I installed the OPA627s on Brown Dogg adapters, added an audio grade 100 uF Nichicon capacitors to each supply input on each opamp (4 in total), and placed a 560 Ohm resistor from output to -V supply to obtain a 9-10 mA bias into class A, which obviously does not cause a heat problem at 5v. The sound seems noticeably improved since before adding the caps and resistor.
Does anyone have any comments / recommendations when operating this opamp at low voltage?
The concern I have is that the OP AMP supply on the 508 is +/-5V, which is less than most other applications I've read.
I installed the OPA627s on Brown Dogg adapters, added an audio grade 100 uF Nichicon capacitors to each supply input on each opamp (4 in total), and placed a 560 Ohm resistor from output to -V supply to obtain a 9-10 mA bias into class A, which obviously does not cause a heat problem at 5v. The sound seems noticeably improved since before adding the caps and resistor.
Does anyone have any comments / recommendations when operating this opamp at low voltage?
Notes on Audio Op-Amps
That link shows the min voltage for a bunch of opamps. OPA627 came in at:
I've tested quite a lot of the opamps from that list in my little dot I+. I have lme49860, lt1364, ad8599, ad823, ad8610 ,opa227, opa2132, opa123, opa627. I found opa627 to have superior detail and balance. Depending on the role of the opamp I/V or gain stage they may work better or worse. I have lme49860 in my I/V and it's good.
His comparison shows AD823, Ad8065, and AD8397 to be the best for low voltage so you might just try those 3 and see which works best.
That link shows the min voltage for a bunch of opamps. OPA627 came in at:
Just a wild idea I had would be you could try building a bigger battery compartment and tie the extra cells directly to the opamps because extra voltage may/may not affect how the transport and dac run. Your other option is to just use other opamps.
I've tested quite a lot of the opamps from that list in my little dot I+. I have lme49860, lt1364, ad8599, ad823, ad8610 ,opa227, opa2132, opa123, opa627. I found opa627 to have superior detail and balance. Depending on the role of the opamp I/V or gain stage they may work better or worse. I have lme49860 in my I/V and it's good.
His comparison shows AD823, Ad8065, and AD8397 to be the best for low voltage so you might just try those 3 and see which works best.
Thanks.
He defines the voltage as rail to rail, though. So when he says 9.8V is needed, my rail to rail voltage is +5.1V - (-5.1V) = 10.2V, which is acceptable.
I haven't noticed any problems - the sound seems very strong through headphones.
But, given the fact that many say this opamp performs better at higher volatges, I'm wondering if anyone would suggest further tweaks.
He defines the voltage as rail to rail, though. So when he says 9.8V is needed, my rail to rail voltage is +5.1V - (-5.1V) = 10.2V, which is acceptable.
I haven't noticed any problems - the sound seems very strong through headphones.
But, given the fact that many say this opamp performs better at higher volatges, I'm wondering if anyone would suggest further tweaks.
What those figures don't show in natek83's link is how the distortion rises as load impedance falls.
No opamp will drive loads of such a low value as 33 ohm correctly, the distortion will increase dramatically, as well as a virtually non existant output voltage swing... even if it seems to sound OK at low levels.
600 ohm is a recognised "minimum" for many devices, and for some older chips such as TLOxx's etc it's far higher than that.
Rather than use a resistor (560 ohms... that's bit on the low side🙂), an active constant surrent source/sink is better, perhaps using 1 or 2 jfets. The current flowing in a resistor is anything but constant as the output signal varies. As the signal approaches the negative rail the current falls away, at the other extreme as it approaches the positive rail the current flowing in the resistor approaches 18ma approx.
I would have thought 3 or 4 ma at most to be sufficient (if you feel the need to bias them in this way in the first place).
No opamp will drive loads of such a low value as 33 ohm correctly, the distortion will increase dramatically, as well as a virtually non existant output voltage swing... even if it seems to sound OK at low levels.
600 ohm is a recognised "minimum" for many devices, and for some older chips such as TLOxx's etc it's far higher than that.
Rather than use a resistor (560 ohms... that's bit on the low side🙂), an active constant surrent source/sink is better, perhaps using 1 or 2 jfets. The current flowing in a resistor is anything but constant as the output signal varies. As the signal approaches the negative rail the current falls away, at the other extreme as it approaches the positive rail the current flowing in the resistor approaches 18ma approx.
I would have thought 3 or 4 ma at most to be sufficient (if you feel the need to bias them in this way in the first place).
I tried the OPA627 in my Meridian 508 to replace the original NE5532s, and they worked at +/-5V. There was improvement, but the OPA627 really wants the full +/- 15V to give best performance. I am now using LT6230, which is happy at +/-5V. In fact there is a whole lot of low-voltage of opamps to choose from these days, like the ADA4841, TS971, etc.
You will need to make a small adaptor board yourself.
Patrick
You will need to make a small adaptor board yourself.
Patrick
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