Specs that aren't on website

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mfsoa

Specs that aren't on website
« on: 26 Feb 2010, 03:47 am »
Hi Tommy,

Can you tell us the following (these questions have popped up and I don't see on the web site)

- Input impedence?

- Power characteristics below 4 ohm. Do the amps continue to increase power into 2 ohm/ level off at 2 ohm/ roll over and beg for mercey at 2 ohm (doubt it).

I know many specs don't mean much but I think that these might help folks in system matching.

Thanks,

-Mike

shadowlight

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Re: Specs that aren't on website
« Reply #1 on: 1 Mar 2010, 06:34 pm »
I am going to piggy back on Mike's question related to impedance.  Can you provide short tutorial on what happens when you have a impedance mismatch, especial between the pre-amp and amp.

Thanks

AmpDesigner333

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Re: Specs that aren't on website
« Reply #2 on: 1 Mar 2010, 10:11 pm »
Input impedance on DAC4800A and all versions of Cherry is about 10K.

As far as a tutorial on impedance matching, see here:
http://en.wikipedia.org/wiki/Impedance_matching

However, matching a preamp to an amp is rarely done these days!  Impedance matching has to do with power transfer, and the idea is to have near zero power transferred in the first place.  For example a 10Vrms sine wave --- a huge signal as far as preamp outputs go --- dissipates only 10mW into 10K.

The real concern with modern equipment is signal loss.  Typically, the lower output impedance of the preamp the better, and the higher on the amp the better.  The problem with high input impedance (on the amp) is noise --- higher input impedance results in more noise from the input stage, then the noise is amplified!

An amp:preamp impedance ratio of 100:1 or larger is a good rule of thumb.  Some say 10:1 is fine, but that results in almost 1dB of signal loss.  1:100 results in less than 0.1dB loss.

Also, we are talking about impedance magnitude here, but impedance is really complex (as in a vector, not meaning “complicated”).  Cable capacitance and length (within reason) become negligible factors with audio range frequencies and practical impedances.  Here’s a good Wiki on impedance in general:
http://en.wikipedia.org/wiki/Electrical_impedance

The only time we have seen the input impedance of our amps com into play is with passive attenuators, where you need to know what load you’ll be driving.  Decent active preamps should have no problem driving loads well below 1K.  Thanks for reading.


shadowlight

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Re: Specs that aren't on website
« Reply #3 on: 2 Mar 2010, 08:57 pm »
Thank you, now time to go read the wiki.

mfsoa

Re: Specs that aren't on website
« Reply #4 on: 3 Mar 2010, 01:27 am »
Tommy,

Isn't it practically impossible to reach the 100:1 ratio, given a 10K input impedence of the amp?

That'd be 100 ohm output for the preamp - Isn't that really low?

I thought the range for preamps was maybe 300 to 1000 -ish?  I looked for the info for my pre and couldn't find it.

Anyway, thanks for the info.

Any info on sub-4 ohm power characteristics (part 2 of my original post)?

-Mike


gerald porzio

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Re: Specs that aren't on website
« Reply #5 on: 3 Mar 2010, 02:34 am »
I've heard a 10:1 ratio touted oft times. There are no immutable ratios. Best is to hear if a demo floats yo boat.

AmpDesigner333

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Re: Specs that aren't on website
« Reply #6 on: 3 Mar 2010, 03:25 am »
Tommy,

Isn't it practically impossible to reach the 100:1 ratio, given a 10K input impedence of the amp?

That'd be 100 ohm output for the preamp - Isn't that really low?

I thought the range for preamps was maybe 300 to 1000 -ish?  I looked for the info for my pre and couldn't find it.

Anyway, thanks for the info.

Any info on sub-4 ohm power characteristics (part 2 of my original post)?

-Mike

Mike,

As far as driver circuits go, sub-0.1 ohm output impedance is not uncommon (closed loop).  In good designs, there is an output resistor added to protect the output against shorting and enable matching if desired.  This output resistor is typically 50 ohms to 600 ohms, so 100 ohms is not too much to ask for.  Here's an example preamp that we use (Audio Research SP-9):
http://www.arcdb.ws/SP9/SP9.html

The main output is 250 ohms.  Ratio wise that's 4x better than the 10:1 "standard".  It sounds very good, by the way.

Interestingly, our amps reveal this preamp's output noise floor even though it's rated at 100dB with respect to 1Vrms.  Our amps have 2.1Vrms input sensitivity, so the 100dB at 1V translates to more than 106dB at 2.1Vrms.  This says a lot about our amps which have enough dynamic range to take advantage of even the best source.

I'll get back to you about sub-4 ohms after some re-measuring...

Sorry if this stuff gets too "engineering centric" for some, but that's audio equipment design for you.

Maybe we should design a preamp (hmmmmm).  Thanks for your kind post.

-Tommy

AmpDesigner333

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Re: Specs that aren't on website
« Reply #7 on: 3 Mar 2010, 04:10 am »
Tested with a Cherry Amp...  Into 2 ohms (resistive load), the current limiter kicks in at about 250W with a 1KHz continuous sine wave.  THD+N is <0.009% until about 100W and <0.02% over 200W.  However, the power output for music signals (transient in nature) is much higher since the hard current limit doesn't kick in instantly.  It's a bit unfair to test these things with continuous sine waves.  Keep in mind that power into 4 ohms is >750W!  We did an experiment playing music (various types) into four 8 ohm speakers in parallel last year, and were not able to hit the current limit even at painfully loud listening levels.  That's a bit subjective, but you get the point.  We don't buckle under heavy load.