In this thread, I suggested a very low O/P impedance buffered volume control feeding a Sowter 1475 transformer. In spite of my being a dyed in the wool "tubehead", a high quality FET I/P opamp seems right for this job. A superior opamp can be adjusted to have zero DC offset at its O/P. That AC only situation is ideal for feeding the 1475, without having to optimize a DC blocking capacitor. I'm thinking in terms of a 7818/7918 regulated bipolar PSU. Please make some suggestions.
You can have a look at the Texas Instruments webside and search for Audio OP´s.
Like these: Audio Op Amps | Overview | Operational Amplifiers | TI.com
Like these: Audio Op Amps | Overview | Operational Amplifiers | TI.com
The classic old FET input opamps had high input offset voltage. Raw FETs do not match as good as raw BJTs.
FET inputs nearly eliminate added offset voltage from base current in input bias resistors.
Times change. Last 10-20 years FET-chip foundries have ion-zap and, for a price, can zap the "high DC accuracy" parts down to microvolts of input error. i.e. less than most of us could measure. That won't be the 19 cent TL072, but I bet a few bucks will buy incredible Vos accuracy with low FET currents.
For audio applications and the gains and configurations used even the humble TL072 you mention will come out with virtually no offset. If you were building precision instrumentation amps from separate devices then you might look closer.
Build a circuit such as I posted using an NE5532 (bjt type) and you could well have a few 10's of millivolts offset, use the TL072 or similar and that would drop to under a millivolt.
Build a circuit such as I posted using an NE5532 (bjt type) and you could well have a few 10's of millivolts offset, use the TL072 or similar and that would drop to under a millivolt.
Maybe I am misunderstanding your requirements 
What will the opamp be driving ? and what will feed the opamp ?
The TL071 is the single version of the TL072 and is a great audio device as long as you are aware of its limitations on 'drive ability. A 2k load is about the minimum if you drive it fully on -/+15 volt supplies. Being a FET device it would also match well to a high impedance volume control feeding it.
Compared to a valve stage 2k probably seems like a short circuit
What will the opamp be driving ? and what will feed the opamp ?
The TL071 is the single version of the TL072 and is a great audio device as long as you are aware of its limitations on 'drive ability. A 2k load is about the minimum if you drive it fully on -/+15 volt supplies. Being a FET device it would also match well to a high impedance volume control feeding it.
Compared to a valve stage 2k probably seems like a short circuit
The '1612's Vos is 0.5mV, like 1/5th that of a TL072 (there's several grades of the '07x parts).
The '1612's input bias current is "huge", like '5532. 60nA typ but 300nA worst-case (which is not specified). Taking a pessimistic view with a whopping 1Meg DC path unbalance this is 300mV offset! Typical with a 100K resistor is 6mV, which still leaves the 'TL072 looking good. The dc path must be nearer 10K to hold the offset down below a mV (for most cases).
The optimum source impedance of '1612 is ~~1K. At 10K (50K pot set half way) the chip hiss is 6dB higher than the resistor hiss (a '5532 would be better for this case).
Though for the intended application, bridging stereo to mono, for my money, as long as there's opamps, I would just throw in an opamp inverter, unless the builder has intense love for transformers.
I'm researching options, at this time.
The fellow that originated the thread linked in my 1st post here is "calling the shots". He wants to convert a tubed stereoblock into a monoblock. Bridging is the 100% safe method for doing that and Sowter's model 1475 is a good way of obtaining the requisite 180o out of phase signals. That item should be driven by a low impedance source and "standing" DC is not allowed. Hence my interest in an opamp.
Perhaps members expert (I'm not) in handling opamps should post in the triggering thread with 100% opamp bridging adapter designs.
The fellow that originated the thread linked in my 1st post here is "calling the shots". He wants to convert a tubed stereoblock into a monoblock. Bridging is the 100% safe method for doing that and Sowter's model 1475 is a good way of obtaining the requisite 180o out of phase signals. That item should be driven by a low impedance source and "standing" DC is not allowed. Hence my interest in an opamp.
Perhaps members expert (I'm not) in handling opamps should post in the triggering thread with 100% opamp bridging adapter designs.
I'm just looking at that thread now and maybe I am still not understanding what you want...
You mention impedances of speakers. My understanding of bridging is that it would not allow a lower impedance speaker to be used, just the opposite in fact because it places greater demands on each driving amplifier with regard to current delivery.
I imagine with a valve amp and transformer output coupling that damping factor would be compromised as well.
You mention impedances of speakers. My understanding of bridging is that it would not allow a lower impedance speaker to be used, just the opposite in fact because it places greater demands on each driving amplifier with regard to current delivery.
I imagine with a valve amp and transformer output coupling that damping factor would be compromised as well.
Not easy to rationalise moving away from FET's when DC is the topic, typically bipolar is more of a price balance choice - precision, low drift FET parts like the OPA2140 are very good at what they do. The 1612 isn't much of a pick for DC specs amongst bipolars though, something like the ADA4075-2 does better in application.
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So is an op-amp. A basic TL072 inverter will be dead-on 180o from DC (whatever that means) out to 100KHz.
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