Discrete Opamp Open Design

Thanks for this hitsware. So Ro = 1/hoe

A quick experiment measuring 1, 2, and 3 matched diodes in parallel gives about 35 Ohms series R so they behave "ideally" only up to 10's of uA.
So my crude estimate of 20R for 1n4148 is in the ballpark and so is the 'cure' .. tweaking the simple current mirror resistors.
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I propose to yus gurus SPICEing that we also do a more stringent load test

3.162Vp 20kHz into 15R. OPA configured as Voltage follower

This is from NwAvGuy: O2 Headphone Amp

I'm having difficulty getting the various contenders to show THD above -140dB on a 600R load which implies no sensible real life results with a 24b soundcard. It will also show up important 'CMR' effects.

It's already showing up some of the BJT models at high currents.
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BTW, IMHO, even with seriously PITA matching, I think we will need tweak pots for offset and maybe for Iq. The first guys who build a breadboard get to pontificate on this. :D
 
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I'm having difficulty getting the various contenders to show THD above -140dB on a 600R load which implies no sensible real life results with a 24b soundcard. It will also show up important 'CMR' effects.

It's already showing up some of the BJT models at high currents.
_______________________________

BTW, IMHO, even with seriously PITA matching, I think we will need tweak pots for offset and maybe for Iq. The first guys who build a breadboard get to pontificate on this. :D

Of course you will need to trim or select resistors if you want really low offset. We also went over the output current thing before, if you want a 32 Ohm headphone amp it will be different.
 
Regarding the follower stage of the open OPA, There are some Sanyo BJT's available in SOT89 (Sanyo PCP) package that are supposedly compliments (although not very well matched) 2SA2125 2SC5964. Data sheet doesn't specify use for linear applications but the transfer function looks pretty good in the range it will be used (<300mA) and they have a fairly good fT ~ 380mHz and a good max dissipation for SOT89.
Would there be any performance advantage in using these when compared to say the BCP53/56 compliments?
 

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Thanks for this hitsware. So Ro = 1/hoe

So my crude estimate of 20R for 1n4148 is in the ballpark and so is the 'cure' .. tweaking the simple current mirror resistors.
_______________________________

I propose to yus gurus SPICEing that we also do a more stringent load test

3.162Vp 20kHz into 15R. OPA configured as Voltage follower

This is from NwAvGuy: O2 Headphone Amp

I'm having difficulty getting the various contenders to show THD above -140dB on a 600R load which implies no sensible real life results with a 24b soundcard. It will also show up important 'CMR' effects.

It's already showing up some of the BJT models at high currents.
_______________________________

BTW, IMHO, even with seriously PITA matching, I think we will need tweak pots for offset and maybe for Iq. The first guys who build a breadboard get to pontificate on this. :D

If you need a low impedance load then maybe you should consider the FET output stage used by Mr Cordell.
 
Scott, I'm really interested what you think about my little baby?
Sonitus, will you oblige us with your LTspice file so us wannabe SPICE gurus can have a play? Also your Rush-Rush version?

I propose to yus gurus SPICEing that we also do a more stringent load test

3.162Vp 20kHz into 15R. OPA configured as Voltage follower
If you need a low impedance load then maybe you should consider the FET output stage used by Mr Cordell.
On 2nd thoughts, 210mA peak may be a bit too stringent. c2240/a970 & bc556/546 are only 100mA. 4401/3 are OK but I was trying to get a series of tests for the contenders which didn't need too many special devices in special places. Special i/ps are unavoidable.

Got a link or sketch of Cordell's output stage accessible to beach bums?

Low loads aren't a problem if you are prepared to use medium power devices like the original 990 or even just 4401/3.

Scott, did you get your breadboard up & running?

My contention about offsets is that you may need tweak pots to avoid Volts of offset rather than mVs even with loadsa device matching. But this is just from SPICE so the breadboard man has the last (??) say.
 
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What is your desired ultimate spec THD into 15R @ 20KHz @ 3.162Vp that you seek to attain.
O2 headphone amp has -90dB THD + noise at that output level using cheapo NJM OPAs.

I'd like to see that with +/- 9V supplies too but that would be too stringent for some of our more exotic topologies. Personally, I like much simpler circuits if they can equal complex stuff. Easier to build, unnerstan and much more reliable. Less quiescent current too if THD remains low and low order like a Class A amp.
 
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O2 headphone amp has -90dB THD + noise at that output level using cheapo NJM OPAs.

I'd like to see that with +/- 9V supplies too but that would be too stringent for some of our more exotic topologies. Personally, I like much simpler circuits if they can equal complex stuff. Easier to build, unnerstan and much more reliable. Less quiescent current too if THD remains low and low order like a Class A amp.

If you want that stuff go for it, I'm not sure that Artero La Mancha wine rings my bell either. If just numbers are what you want the cheap NJM op-amps are fine.
 
If you want that stuff go for it, I'm not sure that Artero La Mancha wine rings my bell either. If just numbers are what you want the cheap NJM op-amps are fine.
Pace Lord Shiva. This is a Discrete OPA thread and I'm having fun trying to beat OPA627 etc and learning from Guru Wurcer at the same time.

But I'm sure you would like to at least equal the numbers of cheapo stuff but also your AD797. I was just answering Pheonix's query.

FWIW, my poor efforts with LTspice, show both SW-OPA & my hacked FET990 meet this spec with 4401/3s .. even w/o increasing Iq. I mention the +/- 9V cos I would like it .. but am not using it as a criteria as it would be unfair.

One shows more xover but with all harmonics below -100dB, I'm not sure I'm going to complain. Gotta figure out an easy way to either show distortion residual or sum THD with something like BBC weighting.

I gather Artero La Mancha is a cheapo wine which is highly regarded. I used to be an inverted wine snob who used to delight in confounding connoisseurs with cheapo bottles in blind tastings against expensive ones. You do have to work hard to find these. But the best red I have ever tasted was a 1975 Volnay.

Alas, this beach bum can no longer afford to drink what he likes, either cheapo or Burgundy.

PS I used to like several Tempranillo wines including one called La Mancha but that was definitely last millenium so dunno if this is the same.
 
If just numbers are what you want the cheap NJM op-amps are fine.
I ask this question in fear & trembling but .. if numbers are not the main criteria for this thread, what is?

Yus gurus, please be frank but gentle to this beach bum.

PS: I did attempt to study the whole thread but I admit to giving up after about 160 pages and skipped to the last pages. Mea maxima culpa. :eek:
 
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I don't go for it much lately, but we did enjoy a 1976 Rumier Bonnes Mares a few weeks ago, but then again I don't think there is a two buck pinot noir.
Roumier, a name that is a distant recollection :) That year btw obtainable today at about 450 US.

One of my top twenty or so was a rebottled 1934 Bonnes Mares (I didn't save the bottle and don't remember the specific maker). That and a couple of other much-too-old burgundies showed up at Vendome, a now-defunct retailer in Westwood. I had two of the Bonnes Mares, and my friend Kathryn still remembers the wine and the evening in 1973. We began with some of a 1964 DRC Echezeaux!

There was a good deal of telepathy in evidence. Certainly an evening for the Journal of Irreproducible Results. We sat on barber chairs which slowly, undetectably, but inexorably, sank towards the center of the planet.
 
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Joined 2005
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I gather Artero La Mancha is a cheapo wine which is highly regarded. I used to be an inverted wine snob who used to delight in confounding connoisseurs with cheapo bottles in blind tastings against expensive ones. You do have to work hard to find these. But the best red I have ever tasted was a 1975 Volnay.

Almost certainly a 76. 75 was quite poor in general.