Modeling differential input pair (audio amplifier) - Page 1
Forum section: electronics / beginners

author: materials_guy
author_rank: Contributor
timestamp: January 03, 2015, 01:31:14 pm
content: v

author: dannyf
author_rank: Super Contributor
timestamp: January 03, 2015, 01:59:11 pm
content: QuoteDepending on what you are trying to understand.

author: materials_guy
author_rank: Contributor
timestamp: January 03, 2015, 03:06:01 pm
content: v

author: materials_guy
author_rank: Contributor
timestamp: January 03, 2015, 03:33:42 pm
content: v

author: dannyf
author_rank: Super Contributor
timestamp: January 03, 2015, 04:58:22 pm
content: What you get is a function of the circuit the diff amp is in.Once the feedback loop is complete, the difference between the two bases is zero.You should try to make a distinction between the diff amp by itself and the diff amp in this circuit. Depending on the context, you may observe different behaviors.

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2015, 11:58:53 am
content: Here is a quick simulation of your circuit, in a closed loop.As you can see, the feedback is almost on top of the input.

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2015, 12:02:16 pm
content: Here is the current through the two legs of the input stage:As expected, they move linearly (due to the feedback) over the whole input range and in opposite direction. In an open loop, they would do the same but over a much narrower range.

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2015, 12:04:35 pm
content: Here is the current through the emitter resistor. It varies (slightly) with the input signal, as we discussed earlier.

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2015, 12:15:37 pm
content: The amp design is very cleaver. Rather than the typical VAS, the input stage is followed by a current amplifier (CAS?). The only thing that comes close to this is THS4120 - see attached.It is the first time I have seen people doing this in audio - JLH1969 may be the only exception.

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2015, 12:35:10 pm
content: I just noticed that this is a Mcintosh? Those guys are good, and apparently quite innovative.R106 is also quite cleaver.The protection mechanism (if it is so) looks rudimentary, however.

author: materials_guy
author_rank: Contributor
timestamp: January 04, 2015, 02:04:07 pm
content: v

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2015, 03:54:17 pm
content: C102 is a DC blocking cap: it allows 100% feedback of DC and some feedback for AC.R106 provides positive feedback to increase the input impedance.D102 Q106 is a current amplifier: its gain set by R114/R116.Even for today's amp, it is an excellent design.

author: materials_guy
author_rank: Contributor
timestamp: January 04, 2015, 04:00:18 pm
content: v

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2015, 04:16:38 pm
content: The input stage, as we discussed earlier, is really amplifying a common mode signal - the feedback loop assures that the other base sees the same signal as the base where the input signal is attached to.The output current in the collector follows the input voltage signal. The collector current is amplified by the D102/Q106, as a result the collector current in Q106 follows the input voltage.

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2015, 05:32:30 pm
content: More amazing about this amp: it actually has output transformers!wow.

author: materials_guy
author_rank: Contributor
timestamp: January 04, 2015, 05:45:29 pm
content: v

author: materials_guy
author_rank: Contributor
timestamp: January 05, 2015, 10:46:44 am
content: Quote from: dannyf on January 04, 2015, 04:16:38 pmThe role of the diode has me puzzled(?). Can you provide insight as to it's operation and it's role in the Q106 amplification?

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2015, 12:22:24 pm
content: Quote from: materials_guy on January 04, 2015, 05:45:29 pmWTF? I read the section on transformers for transistor amplifiers and it looks like a gimmick.There's no need for a transformer with a properly designed transistor amplifier. It should be able to cope with the speaker's impedance changing. If the 8 Ohm speaker is 4 Ohm at some frequencies (which I doubt) it's not that low all of the time, at other frequencies it'll be a higher impedance than 8 Ohm so it'll balance it out.If the amplifier starts to overheat because the speaker impedance is lower than 8 Ohm at some frequencies, then it's because it's poorly designed or it's been abused by someone connecting a signal generator to it and adjusting the frequency for maximum power dissipation.The other reason for a transformer in a transistor amplifier is if it's a PA system which distributes the audio at 100V and transformers are required to reduce the voltage to drive 8 Ohm speakers.

author: dannyf
author_rank: Super Contributor
timestamp: January 05, 2015, 06:33:22 pm
content: On the diode - there are two ways to calculate the voltage on q106 base, one is through the diode side, where it equals the diodes forward voltage drop plus the input stages collector current time 560ohm.Another way is vbe of q106 plus its emitter current time 22ohm.The two voltage must be equal, and the diodes forward volt drop is about the vbe so q106 emitter, thus its collector, current is proportion to the input stages collector current.In reality, they aren't exactly proportional but the negative feedback will take care of that.

author: materials_guy
author_rank: Contributor
timestamp: January 05, 2015, 11:10:42 pm
content: v

author: materials_guy
author_rank: Contributor
timestamp: January 10, 2015, 11:23:32 am
content: Quote from: Hero999 on January 05, 2015, 12:22:24 pmThe folks at McIntosh wouldn't disagree that a properly designed amplifier should be able to cope with a speakers varying impedance. They've made 10s of thousands of amps without autoformers. The impedance on some speakers can have impedance ratings lower than the so-called nominal rating. A 4 Ohm nominal impedance can really be closer to 2 Ohms at some frequencies. Crossover networks that provide frequency contouring can cause this. So autoformer is good choice. Adds considerable cost, weight and size to the amp however.

author: materials_guy
author_rank: Contributor
timestamp: January 10, 2015, 11:40:31 am
content: v

author: dannyf
author_rank: Super Contributor
timestamp: January 10, 2015, 12:29:22 pm
content: QuoteIt provides comparable power output for a wide range of loads.And I think they add the desirable "color" / sound signature / distortion to the amplifier.

author: dannyf
author_rank: Super Contributor
timestamp: January 10, 2015, 12:38:31 pm
content: QuoteThe diode (and the resistor associated with it) is not needed for the circuit to work, albeit at much higher gain.The diode (and I wouldn't be surprised if in production it is of the same type as Q106) does a few things:1) it makes the gain better defined by offsetting Q106's Vbe;2) it provides tempco compensation;3) it lowers the gain;4) it makes the gain independent of the supply rails.The end result is a fast amp with lower and more linear open loop gain, thus low levels of feedbacks are needed for this amp. That's something people didn't discover until the 1990s and early 2000s. You will also notice that the feedback resistors are of much lower values vs. a typical design,. If you compare the mcintosh vs. other amps from the same era (Citation 12 for example), this one blows their doors open.So the amp, if it is a 50-yr old design, is quite ahead of its time and quite innovative. The only thing I would change, maybe, is to use truly complimentary devices. The Motorolla 2119x are very good, as the Toshiba 1943/5200 (or if you could find their older brother / sistor 1302/3281).

author: dannyf
author_rank: Super Contributor
timestamp: January 10, 2015, 12:53:57 pm
content: I haven't seen this but I would venture a guess that this amp doesn't have too impressive of a thd spec?that's a sure sign of a quality amp - necessary but not sufficient,

author: materials_guy
author_rank: Contributor
timestamp: January 11, 2015, 12:04:52 pm
content: v

author: dannyf
author_rank: Super Contributor
timestamp: January 11, 2015, 12:59:44 pm
content: The beta will change with bias Ic. So without a defined amplification factor, your current gain would change as well.

author: GK
author_rank: Super Contributor
timestamp: January 12, 2015, 12:16:43 am
content: Quote from: dannyf on January 04, 2015, 12:15:37 pmThis amplifier isn't very "cleaver" at all. It is in fact very rudimentary circuit completely typical of the era. That diode is just a lame and relatively ineffectual attempt to compensate for the VAS transistors Vbe and force current balance in the long tail pair. The diode doesn't change the fundamental operation of the 2nd stage - the stage is still in every sense a "VAS".The "2nd" stage of the THS4120 is hardly unique. It is known as a "folded cascode" and has been used in integrated op-amps for decades (the AD797 is the most popular "audio" op-amp to use the topology). It is in no way analogous in operation to the amplifier circuit presented here. The folded cascode "VAS" simply mirrors the long tail pair signal current, transposed onto a higher value quiescent current. It is in fact a common-base stage that mitigates the Miller effect and actually has a signal current gain of one. It is for the latter reason that the combined 1st and 2nd stages of amplifiers using the topology are often lumped together and referred to as a single gm/gain stage/block.