On the 797
The 797 is Professor Gerry Sussman's favorite opamp. He's one of the legendary EECS profs over at the 'Tute, and he is into audio somewhat (although, you can imagine he leans on the "numbers" side of the objectivist/subjectivist scale) and apparently collaborates with friends of his at Caltech. I think he's used this opamp to make some ridiculously good mic pre-amp, but I've never seen the schematics.
Also, a friend of mine recently did distortion comparisons between various gain devices, including various BJTs, FETs, Triodes, Pentodes and Opamps such as the 797. The 797 "won" in the numbers category, having low "gain normalized" distortion all around (primarily of low, even orders).
-Won
The 797 is Professor Gerry Sussman's favorite opamp. He's one of the legendary EECS profs over at the 'Tute, and he is into audio somewhat (although, you can imagine he leans on the "numbers" side of the objectivist/subjectivist scale) and apparently collaborates with friends of his at Caltech. I think he's used this opamp to make some ridiculously good mic pre-amp, but I've never seen the schematics.
Also, a friend of mine recently did distortion comparisons between various gain devices, including various BJTs, FETs, Triodes, Pentodes and Opamps such as the 797. The 797 "won" in the numbers category, having low "gain normalized" distortion all around (primarily of low, even orders).
-Won
I am not a big fan of the OPA-2132 and 2134 opamps for High End Audio. These devices are very stable and indeed are alot better than the opamps used in most Equipment like the LM-883 and the NE5532. But are Vailed and have more Grain to the Mids than i can tolerate. for Better performance and sound quality i would go with the AD-823 fast and Articulate. or the AD-746 this is a Dual AD-744 and is Nice and warm sounding. However dark sounding compaired to the AD-823. Analog Devices have some New opamps that look good but i havent tried them yet the AD-85XX series. If you would consider single opamps then your choices open up alot as atated in your Post.
BTW the AD-797 is good for High closed loop gains like Mic and Mag phono, However like the LT-1028 another Good device for High Closed loop gains. The 797 is unity gain stable but it's high bandwidth and low slew rate could have Problems with EMI and the Feedthrough from the Clock on Digital sources. However lots of folks like the 797 for line level app's but not my cup of tea for sutch use.
Regarding not Biasing the OPA-627 into class A using a buffer. I tried Both Ways using a BUF-634 X 3 as an output stage and the Current source improved the sound alot with all Opamps, But was a great help for the 627. you only need a Ma or two.
BTW the AD-797 is good for High closed loop gains like Mic and Mag phono, However like the LT-1028 another Good device for High Closed loop gains. The 797 is unity gain stable but it's high bandwidth and low slew rate could have Problems with EMI and the Feedthrough from the Clock on Digital sources. However lots of folks like the 797 for line level app's but not my cup of tea for sutch use.
Regarding not Biasing the OPA-627 into class A using a buffer. I tried Both Ways using a BUF-634 X 3 as an output stage and the Current source improved the sound alot with all Opamps, But was a great help for the 627. you only need a Ma or two.
richt said:
You're definitely right about it being tougher to repair a surface mount board (especially if you don't have the right equipment, which is expensive). The reality is that the manufactures don't really want you to repair the boards yourself anymore (if they ever did). Surface mount just makes that less likely.
It is hard to imagine why an op amp would blow up that violently. Maybe someone plugged that mic input into a wall outlet? Personally I consider Mackie mixers disposable, I don't have time to take all of those knobs and screws off everytime you need access to the PCB. Give me a mixer with individual channel strip PCB's like A&H any day.
By the way, when you guys made the replacement PCB, did you use the same grade of components? I know that Mackie uses 47 uF electrolytics as DC blocking caps in the signal path on their mic preamps. It isn't hard to improve the sound of that preamp by putting better quality caps in (makes their marketing campaign about how good the XDR preamp is that much more laughable).
Phil
I will let you know how it turns out. I also plan to build a current buffer amplifier as well, although I think instead of the Szekeres I am going to try a discrete buffer similar to that published by Walt Jung:
www.planetee.com/planetee/servlet/DisplayDocument?ArticleID=2800
I hope to use that for my lower-impedance headphones like the ER-4P, and I'll use the OPA627/BUF634 amplifier for my Sennheisers.
The AD797 seems to be, on paper, the best availble in most areas, esp noise and distortion. It can drive +/- 50mA (Within its power dissipation limits for its package).
The downsides are, as mentioned, it is quite prone to oscillation, and ideally must have a low as possible drive impedance on its input, it's own noise contribution is so low that an input imedance of less than 1k is suggested, otherwise the thermal noise of the resistor will swamp the opamps noise.
Cheers, Adrian
The downsides are, as mentioned, it is quite prone to oscillation, and ideally must have a low as possible drive impedance on its input, it's own noise contribution is so low that an input imedance of less than 1k is suggested, otherwise the thermal noise of the resistor will swamp the opamps noise.
Cheers, Adrian
Thanks for your comments, I will definitely keep this is mind. In fact, I guess what I will probably do is initially try the circuit without the current source but provide for adding it later so I can evaluate the difference. Could this difference possibly be due to changing the OPA627's output stage from complementary to single ended?
hi haldor
with mackie design we dont have enough space to put better caps on the pcb.
the smd design creates a lot of crosstalk specially when a channel is clipping.
the .1 caps are very low & leaky (audio voltage rails) drawing a lot of current from power supply & making smaller burned holes on pcb.
i stll prefer old designs & i love tubes.
a 14 channel pcb (mackie) cost $475.00 us if you cant repair 1 blown channel
thanks
richt
with mackie design we dont have enough space to put better caps on the pcb.
the smd design creates a lot of crosstalk specially when a channel is clipping.
the .1 caps are very low & leaky (audio voltage rails) drawing a lot of current from power supply & making smaller burned holes on pcb.
i stll prefer old designs & i love tubes.
a 14 channel pcb (mackie) cost $475.00 us if you cant repair 1 blown channel
thanks
richt
another new op amp ...
I got data sheets and samples of a couple of new op amps today. No chance to play yet.
1. The previously mentioned AD8610. Looks to be designed as a OPA627 killer - even has head to head performance comparsions in the data sheet. JFET inputs on Analog's high speed process. BUT limited to +/- 13V rails. VERY low THD, low noise, high DC precision, great settling time, etc.
2. AD8021. Bipolar inputs also built on Analog's high speed process. This part is intriguing because it has a feature I've only seen on the AD744 - the ability to take the output prior to passing through the output stage, so one could try Walt's slick trick of using this device's front end driving the buffer of your choice for a nice line stage. I haven't read the data sheet completely, but it seems you can also turn off the output stage.
Michael
I got data sheets and samples of a couple of new op amps today. No chance to play yet.
1. The previously mentioned AD8610. Looks to be designed as a OPA627 killer - even has head to head performance comparsions in the data sheet. JFET inputs on Analog's high speed process. BUT limited to +/- 13V rails. VERY low THD, low noise, high DC precision, great settling time, etc.
2. AD8021. Bipolar inputs also built on Analog's high speed process. This part is intriguing because it has a feature I've only seen on the AD744 - the ability to take the output prior to passing through the output stage, so one could try Walt's slick trick of using this device's front end driving the buffer of your choice for a nice line stage. I haven't read the data sheet completely, but it seems you can also turn off the output stage.
Michael
I have been Evaluating the AD-8510 for a few days in A Portable Headphone Amp Circuit using Walt Jung's Multiloop Feed back and an EL-2008 Open Loop Buffer. A OPA-627 and AD-825 have been in this Amp untill now. The AD-8010 is the Most Grain free Opamp i have run across. Makes the OPA-627 sound Tizzy and I was always impressed with the 627's clarity untill Now. The 8610 also lacks the Prominate Foward Midrange of the 627 Yet still has all the MicroDynamics of the 627. The Bass Extention is Better than the AD-825 and I liked that quality of the 825 untill i heard the8610. so Ya get some 8610's if your are going for an Opamp Based Line Level stage. In fact this OPA outperforms "Sonicly quite a few Discreet component Line Stages. Overall I think Analog devices has outdone themselvs on this for an Audio Opamp.
You are right phil. Still without compensation and a gain of 1 it will not drive more than ~30pF safely. Gain of 10 then this is raised to about ~100pF.
If you wan't to drive larger load than this you have to make the compensation shown in figure 39. Where the 33 Ohm resistor can be replaced by a buffer. Yes then it will drive up to 5nF at again larger than 2.
Sonny
If you wan't to drive larger load than this you have to make the compensation shown in figure 39. Where the 33 Ohm resistor can be replaced by a buffer. Yes then it will drive up to 5nF at again larger than 2.
Sonny
Did a search on this forum for info on Linear Technology opamps and buffers, found mention of the voltage regulators only.
I was hoping that some of you folks out there in DIY-dom has used and listened to these opamps and buffers.
I was searching through my small corner of the garage were all my parts, chassis, heatsinks, and unfinsihed projects are and discovered a box with all the parts to build a line stage and mc/mm preamp using LT devices from an old issuse of TAA. Gary Galo tested these opamps and he liked them alot. Every where I read seems to be about the AD, OPA, and BUF devices. I know Welbrone Labs sells a kit using the LT-1010 for a long while but never hear it.
I was hoping that some of you folks out there in DIY-dom has used and listened to these opamps and buffers.
I was searching through my small corner of the garage were all my parts, chassis, heatsinks, and unfinsihed projects are and discovered a box with all the parts to build a line stage and mc/mm preamp using LT devices from an old issuse of TAA. Gary Galo tested these opamps and he liked them alot. Every where I read seems to be about the AD, OPA, and BUF devices. I know Welbrone Labs sells a kit using the LT-1010 for a long while but never hear it.
OPA132 and 134 the same!?!
I don't think it has been pointed out anywhere: the OPA132 and 134 are the same op-amp - have a look at the data sheets.
The 134 is marketed specifically for audio, comes only in one grade, the typical specs of which are identical to the lower grade of the 132. In terms of maximum offset drift etc. the 132 lower grade is still superior, but at twice the price.
Looking at the disto plots, especially at 10-20 kHz, the 132 looks even better. Whether this is due to typically better offset matching or just of coincidence of which samples were measured I can't tell.
Eric
I don't think it has been pointed out anywhere: the OPA132 and 134 are the same op-amp - have a look at the data sheets.
The 134 is marketed specifically for audio, comes only in one grade, the typical specs of which are identical to the lower grade of the 132. In terms of maximum offset drift etc. the 132 lower grade is still superior, but at twice the price.
Looking at the disto plots, especially at 10-20 kHz, the 132 looks even better. Whether this is due to typically better offset matching or just of coincidence of which samples were measured I can't tell.
Eric
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