Technics Class-AA / Sandman Bridge

[richie00boy]

Does this type of amp/topology rely on driving a loudspeaker of impedance within a narrow range?

I am going to use one of these amps in a test setup to measure Thiele-Small parameters, but as I will be driving the load via a 2k resistor, I'm wondering if the amps output will be skewed in some way because it's not driving the proper amount of load.

[janneman]

Rich,

If you want a 2k series resistance you don't really need the amp, just drive it from your audio oscillator, or if you use a test CD, from the preamp (if it's solid state; tube preamps generally can't handle that kind of load).

If you really need/want to use the amp, you can always load it with say 8 ohms in addition to the 2k + speaker test load. The amp won't notice the difference.

Jan Didden

[johnnyx]

From what I understand of it, if the load impedence is 2k, then you just need the Class A stage. The high current Class C stage will not be turned on much by the load current. It works like a Wheatstone Bridge in series with the load, so that when the load current is sufficient to unbalance the bridge, the Class C stage supplies the load current while the Class A stage fills in the missing bits of the crossover region. At the same time, the Class C stage increases the effective load impedence of the Class A stage, reducing the distortion.
Why not use a Class A amp for such a load?

[sreten]

Originally posted by richie00boy
Does this type of amp/topology rely on driving a loudspeaker of impedance within a narrow range?

I am going to use one of these amps in a test setup to measure Thiele-Small parameters, but as I will be driving the load via a 2k resistor, I'm wondering if the amps output will be skewed in some way because it's not driving the proper amount of load.

Simple answer is no it doesn't and no it won't.

FWIW technics class AA is not a sandman bridge, it uses a class
B power amplifier to drive the centre point of a 15-0-15V high
current class A amplifiers floating supplies.

sreten.

[richie00boy]

Thanks for all the great (and quick) replies.

Unfortunately my oscillator will only output up to 0dB and in order to generate 100mV across my loudspeaker (at 10 ohms impedance) then I need to swing 22V from my amp.

I would just use a class-A amp as the current requirement is very low, and I have thought about building one as I will be testing drivers a bit. I've also thought about bridging NE5532, run at 18V split this would just about give me enough swing. However, I would like to do some testing today so using my amp seemed like a good choice.

sreten, I thought that what you had described there is the Technics 'New Class-A' topology, not the Class-AA which I thought was based on Dr Sandman's circuit.

[sreten]

Originally posted by richie00boy
sreten, I thought that what you had described there is the Technics 'New Class-A' topology, not the Class-AA which I thought was based on Dr Sandman's circuit.

As fars as I'm aware "New class A" is "Super class A" is Technics "class AA".

You could always use an op-amp configured with currentfeedback for an output impedance
of 2kohm, current capability (NE5532 should be fine) would then be the only issue.

sreten.

[richie00boy]

When the amp was a current model, the lower ones in the range were badged New Class-A. I thought they were different from my Class-AA, maybe it was just a marketing trick.

As the 2k resistor is really only to mimic constant-current operation, I have also thought about just making a cc amp using an op-amp buffered by small-signal transistors. That way I would only need to swing less than a volt.

[djk]

"I have also thought about just making a cc amp using an op-amp buffered by small-signal transistors. That way I would only need to swing less than a volt."

Best solution yet.

[janneman]

Yes, there is a neat circuit to do this called a "Howland current pump", works with any standard opamp and a couple of resistors. Google should lead you to it.

Jan Didden

[janneman]

....http://eportal.apexmicrotech.com/mai...m_id=&f_page=5

Jan Didden