[andyr]

I was wondering whether anyone has experienced a severe rolling off of high frequencies - say, 15Khz and above - with a mosfet amp?

I did some measurements on my mosfet amp which drives 2 ohm tweeters (in an active setup).  To my surprise, the output at 16Khz was only 80% of the output at 6Khz (for a constant input) ... and the output at 20Khz was down to 40%.    (This is with the 2 ohm tweeter connected.)

I have read some people - who perhaps are biased towards BJT amps    - call this the "mosfet power envelope" problem.  Has anyone else experienced this kind of problem?


Thanks,
Andy

[FullRangeMan]

You used music or sinusoidal signal of that freq?
If test was w/music is rare a musical piece has any fundamental freq above 5kHz only hamonics unless its church organ, over 10kHz even presence of harmonics is rare.

[andyr]

You used music or sinusoidal signal of that freq?
If test was w/music is rare a musical piece has any fundamental freq above 5kHz only harmonics unless its church organ, over 10kHz even presence of harmonics is rare.

I used sine waves from my sig-gen.  (Sorry, should have noted that.)

Andy

[G Georgopoulos]

I was wondering whether anyone has experienced a severe rolling off of high frequencies - say, 15Khz and above - with a mosfet amp?

I did some measurements on my mosfet amp which drives 2 ohm tweeters (in an active setup).  To my surprise, the output at 16Khz was only 80% of the output at 6Khz (for a constant input) ... and the output at 20Khz was down to 40%.    (This is with the 2 ohm tweeter connected.)

I have read some people - who perhaps are biased towards BJT amps    - call this the "mosfet power envelope" problem.  Has anyone else experienced this kind of problem?


Thanks,
Andy

roll off has nothing to do with impedance of load,mosfets have high input capacitance that causes this,never the less if mosfet amps are driven hard there is no problem there.

cheers 

[andyr]

roll off has nothing to do with impedance of load,mosfets have high input capacitance that causes this,never the less if mosfet amps are driven hard there is no problem there.

cheers 

Sorry, I don't understand what you are trying to say.

When my mosfet amp is plugged into an 8 ohms load ... it has no problem delivering 30Khz.

However, when it is plugged into my 2 ohm ribbons ... its output at 30Khz is only 40% of its output at 6Khz.  To my way of thinking, that makes it pretty useless for a tweeter amp! 

Andy

[G Georgopoulos]

Sorry, I don't understand what you are trying to say.

When my mosfet amp is plugged into an 8 ohms load ... it has no problem delivering 30Khz.

However, when it is plugged into my 2 ohm ribbons ... its output at 30Khz is only 40% of its output at 6Khz.  To my way of thinking, that makes it pretty useless for a tweeter amp! 

Andy

sounds like a lousy tweeter mate...
mosfets bw goes up to 100khz when designed right
sorry i can't say more (proprietary technology)

[andyr]

mosfets bw goes up to 100khz when designed right

Good to hear, GG! 

Andy

[JohnR]

Is the amp actually rated to drive a 2 ohm load?

[andyr]

Is the amp actually rated to drive a 2 ohm load?

Supposedly, yes, John. When I upgraded from my previous ribbon amp, the designer said that it would be 'able to handle' 2 ohms ... but would get warm.

Andy

[G Georgopoulos]

Is the amp actually rated to drive a 2 ohm load?

Hi John

Mr MOSFET NP look at last feature of this link:
https://passlabs.com/products/amplifiers/integrated-series

[Speedskater]

Were you measuring at the amplifier terminals or at the loudspeaker terminals?

[Davey]

Andy,

Are you positive your measuring scheme is without issue?
Your generator is not rolling off at high frequencies?  Your voltmeter is not rolling off at high frequencies?  Etc, etc, etc.
Do you have 2 ohm and 8 ohm resistors you can measure with vice a loudspeaker load?

You could also try making a measurement with no load connected......to establish a reference.

I the roll-off is indeed real, then you have a either a poorly designed and/or defective amplifier.

Dave.

[FullRangeMan]

You could also try making a measurement with no load connected......to establish a reference.

I would add making a measurement with a 8 ohms load.
2 ohms is more a short circuit than a load.

[Davey]

2 ohms is more a short circuit than a load.

Not really.
2 ohms is much greater than the output resistance of this amplifier so a simple, low-power, 2 ohm measurement at high frequencies should not cause any decent amplifier a problem.  If Andy's significant roll-off is indeed real it implies there would be a significant roll-off with a 4 ohm load as well.

That said, I suspect this is a measurement issue.

Dave.

[FullRangeMan]

Not really.
2 ohms is much greater than the output resistance of this amplifier so a simple, low-power, 2 ohm measurement at high frequencies should not cause any decent amplifier a problem.  If Andy's significant roll-off is indeed real it implies there would be a significant roll-off with a 4 ohm load as well.

That said, I suspect this is a measurement issue.

Dave.

2 ohms for audio is suicidal, I had a diode fried in a Hartke HA3200 basshead w/a 4 ohms pro FR speaker. I alert that operating at 2 ohms can lead to great emotions.

[Davey]

The suicidal rhetoric is unnecessary.

I'm talking about a quick simple low-power test to confirm (or not) this HF roll-off of Andy's amp.  I'm NOT talking about the long-term normal power operational characteristics when dealing with a real load year in and year out.

Let's stay focused please.

Dave.

[andyr]

Were you measuring at the amplifier terminals or at the loudspeaker terminals?

I was measuring AC voltage with a multimeter on the driver input terminals - I also had my CRO connected to the amp's input terminals, so I could adjust the sig-gen output to make sure it was outputting the same signal level at all frequencies.

Andy

[andyr]

Andy,

Are you positive your measuring scheme is without issue?
Your generator is not rolling off at high frequencies?  Your voltmeter is not rolling off at high frequencies?  Etc, etc, etc.

Dave.

Hi Dave,

I have my CRO on the amp's input terminal, to make sure the sig-gen inputs the same voltage (60mV) to the amp, at all frequencies tested.

Not sure how the multimeter (set to the 2v scale) could be "rolling off at high frequencies" ... all it's doing is measuring the voltage at the ribbon input terminals - so shirley it doesn't care what frequency the input signal is?

Do you have 2 ohm and 8 ohm resistors you can measure with vice a loudspeaker load?

You could also try making a measurement with no load connected......to establish a reference.

I do have suitable 2 & 8 ohm reses ... but my thinking was that it is better to use the real (driver) load ... as the ribbon may have a capacitative element which means it's a harder load to drive than a straight resistor?

I the roll-off is indeed real, then you have a either a poorly designed and/or defective amplifier.

Poorly designed!      

Andy

[Speedskater]

First using an old fashioned CD player, double check your DMM at the output of the player using a test tone CD.
Then measure at the output terminals of the amp with no load then 8 Ohm and 2 Ohm loads. Finally the loudspeaker but keep the probes on the amps terminals. Found that out decades ago with an industrial amp that drove a shaker table.

[Davey]

Andy,

Many AC voltmeters don't have flat response to 20khz.  Many don't have response above approximately 400Hz.
You can check yours as Speedskater suggested by attaching directly to your signal source to verify it.

If your speaker load is not resistive then you should set it aside for the time being.
Check your amplifier only with load resistors and/or with no load, so you can characterize it first before you create a complex load by attaching a speaker. 

Dave.