[Brian Cheney]

I think it would be useful to publish scope photos of the output waveforms of the DAC amplifiers, as I suggested in another thread.

I have seen some truly bizarre waveforms from switching amps, all supersonic but all capable of folding down into the audio range.

[AmpDesigner333]

I think it would be useful to publish scope photos of the output waveforms of the DAC amplifiers, as I suggested in another thread.

I have seen some truly bizarre waveforms from switching amps, all supersonic but all capable of folding down into the audio range.

Regarding "folding down", this is called aliasing and is explained by sampling theory.  Here is a basic explanation:
http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem#Aliasing

Aliasing only happens when the waveform is sampled, which is not the case for a switching amplifier output driving a loudspeaker.  The time domain output waveform of a switching amplifier (after the output filter) has switching ripple superimposed on top of the audio.  The ripple is ultrasonic, so it is not heard.  The mechanical limitations of loudspeakers do not allow this superimposed waveform to enter the room, not that it would matter anyway, as long as the switching frequency (Fsw) is high enough (>100KHz).  Most switching amps change states (switch) at a rate of 350KHz or greater.  Higher switching rates cause additional EMI and lower efficiency, and lower rates are harder to filter.  The power dissipated in the loudspeaker by the ripple is typically a few milliwatts.  In order to accurately measure the performance of a switching amplifier, a filter that removes most of the ripple is usually placed between the amplifier under test and the measurement device.  Such a filter requires flat response out to 20KHz or greater, but a rapid fall-off by the 100KHz range.  Attempting to measure a switching amplifier without such a filter can drive the measurement equipment nuts as it tries to sync to the amplifier's output ripple instead of the audio signal.

We are taking some time off for the holiday weekend, but we plan to take some scope shots next week.  Thanks for your post.

[Brian Cheney]

Using the older Audio Precision measurement gear one has the option of switching on 40kHz and 80kHz filters.  Even with the lower filter engaged some poor behavior was noticed from a very well regarded Class D amp.

The new AP stuff has brickwall filters which may well reveal less of this.  In any case the Class D amps I have tried so far either made me uncomfortable or quite ill.

B Cheney
Pres VMPS Ribbon
www.vmpsaudio.com

[jhm731]

DAC4800A is in da house.



PS- I find it interesting that Mr.Cheney would like to see scope photos of Tommy's amps, yet I've never
seen him post any measurments of his speakers. If I'm wrong, please direct me to the measurements.

[AmpDesigner333]

Using the older Audio Precision measurement gear one has the option of switching on 40kHz and 80kHz filters.  Even with the lower filter engaged some poor behavior was noticed from a very well regarded Class D amp.

The new AP stuff has brickwall filters which may well reveal less of this.  In any case the Class D amps I have tried so far either made me uncomfortable or quite ill.

B Cheney
Pres VMPS Ribbon
www.vmpsaudio.com

The AP's built-in filters are in the measurement path but not the PLL sync path, at least as of 2005 or so.  That's most likely the problem.  You probably need an external filter even with the newest of AP analyzers.  Have you tried the AUX-0025?

By the way, your speakers are awesome!  Maybe we can arrange for you to try the DAC4800A or Cherry.  Can you provide impedance plots for any of your designs?  This is just out of curiosity since these amps aren't "afraid" of difficult loads.  I'm a big fan of ribbons and currently have an Apogee Slant-6 hooked to a DAC4800A.  This is driven with an Audio Research preamp and Theta D/A.

I want you to know it is an honor to be corresponding with you!  Thanks again.

[pbrstreetgang]

Yum! I will be real interested to hear your opinion of the DAC vs the TacTs. Any internal shots?

[KS]

AmpDesigner333

A couple of comments from an AudioCircle member and audiophile consumer....

1--Post using your name.  Many (most?) manufacturers do that, including Brian Cheney, Lee of CryoParts, Danny Richie of GR Research, Hugh Dean of AKSA, and others, not to mention the eponymous company names.  You will sell far fewer of your amps, whatever their quality, if you don't have the trust of people here and elsewhere.

2--Put your business address on your web site.  Make it look like a real business, even if it is in your basement at home.

3--What's with your return policy?...unopened?...how can I try your unit.  Reconsider how you count the days of the return policy.  I want to know how many days I can sample an item from the day the delivery company records its delivery to me to the day I ship it back to the vendor.  Make it 20 days or 30 days or whatever you choose to be days in my possession, without making me liable for freight delays.

4--Establish a demo policy.  Have a unit that anyone can pay full price, use for a few days, pay shipping to the next user (who pays full price), and the first user gets a full refund.  It'll eventually get banged up, and you sell that one at cost.  If folks liked it, they'll buy shiny new ones from you at your retail price.  Or, establish any other demo policy that gets your units out for trials.

[opnly bafld]

PS- I find it interesting that Mr.Cheney would like to see scope photos of Tommy's amps, yet I've never
seen him post any measurments of his speakers. If I'm wrong, please direct me to the measurements.

Speaker measurements can be important, but measuring speakers and measuring electronics are two very different things.


I find it interesting that there are people I know that have never heard a class D amp they like and also don't care for DSD recordings. Placebo? maybe or is there something amiss in the frequencies above 20k that in some way affect the audible frequencies? 
http://stereophile.com/features/404metrics/

Lin

[AmpDesigner333]

PS- I find it interesting that Mr.Cheney would like to see scope photos of Tommy's amps, yet I've never
seen him post any measurments of his speakers. If I'm wrong, please direct me to the measurements.

Speaker measurements can be important, but measuring speakers and measuring electronics are two very different things.


I find it interesting that there are people I know that have never heard a class D amp they like and also don't care for DSD recordings. Placebo? maybe or is there something amiss in the frequencies above 20k that in some way affect the audible frequencies? 
http://stereophile.com/features/404metrics/

Lin

Thought I was going to take the weekend off, but...

I read the article (again) and do believe in bandwidth beyond 20KHz.  However, this is a separate subject than output ripple due to the much higher (10x) frequencies involved.  Regarding DSD/SACD, the noise shaping involved encroaches on the "extra octave" above the audible spectrum, so your could be onto something there!

Thanks for your post.

[Brian Cheney]

jhm731:

In fact, some of our speaker measurements (RM/X bass system FR plus distortion) have been published here, and were featured in the "photo gallery" just two days ago.  Not my photo gallery, but the pix that pop up on the right side of the home page every time one accesses this site.

I must also point out that speaker measurements are easily manipulated, falsified, and downright forged.  I have caught at least one manufacturer doing so right here on AC.
Dick Olsher published measurements of our 626Jr in his enjoythemusic.com review that lead to their awarding us a 2008 "Product of the Year" in the Loudspeaker category this week.  Any measurements I would publish would look rather different, particularly in the bass where room interaction comes in play (I make my bass measurements ultranearfield).

As for trying out one of the DAC switching amps, I have no objections.  Warning: I would publish not only my listening impressions, but full test results (FR, distortion and noise spectra, in and out of band behavior) including scope photos that might disturb some audiophiles. 

B Cheney
Pres, VMPS Ribbon
www.vmpsaudio.com

[bummrush]

Well,,, let the games begin!

[lonewolfny42]

jhm731:

In fact, some of our speaker measurements (RM/X bass system FR plus distortion) have been published here, and were featured in the "photo gallery" just two days ago.  Not my photo gallery, but the pix that pop up on the right side of the home page every time one accesses this site.

Their here..... 

[jhm731]

jhm731:

In fact, some of our speaker measurements (RM/X bass system FR plus distortion) have been published here, and were featured in the "photo gallery" just two days ago.  Not my photo gallery, but the pix that pop up on the right side of the home page every time one accesses this site.

Their here..... 

What do the measurements look like above 200hz?

[JohnR]

Could I request please that all discussion (and/or bickering) about speaker measurements be moved to a separate thread, to avoid endangering AmpDesigner's thread. Thanks.

JohnR

[AmpDesigner333]

This scope shot shows the DAC4800A driving a 4 ohm load with a 1KHz square wave (10 watts).  The upper trace is the output waveform showing the switching ripple.  This is a sine wave (approximately 400KHz) and looks like "fuzz" on the top and bottom of the signal.  The lower trace shows this signal after passing through an AP AUX-0025 switching amplifier measurement filter.

I'm working on getting the trace colors to come out different from the scope.  I know these colors suck, but that's what the scope put out.  Once I get this down, I'll publish more scope shots (maybe even some FFTs).  Thanks.

[AmpDesigner333]

Could I request please that all discussion (and/or bickering) about speaker measurements be moved to a separate thread, to avoid endangering AmpDesigner's thread. Thanks.

JohnR

John,

Good idea.  Please suggest one or let me know if you want me to start a new one.  Thanks as always.

Warm Regards,
Tommy

[AmpDesigner333]

We have a DAC4800A demo unit available for $900 (list price is $3600) to AC members.  If you are interested, please email DacSales@DigitalAmp.com.  This unit has a small broken weld in the chassis, but the cover holds it secure (wouldn't notice unless you knew).  The insides are updated to the latest hardware, but the chassis is the old type with input and output connectors on the opposite side as on the new version.  The old style chassis was also made from 18 gauge steel, and the new one is 16 gauge.

Here are details about this amp (new version):
http://www.digitalamp.com/4800a_amp1.htm

Thanks.

[AmpDesigner333]

I'd like to know if anyone is using unbalanced inputs and a "ground cheater plug" to eliminate noise...

[THE_ANSWERS]

interesting topicmaybe something you want to comment on..................

http://www.audiocircle.com/index.php?topic=50955.msg523750;topicseen#new

on another subject    the amp you had on ebay didnt sell     you are barking up the wrong tree on ebay and amazon    yoshould try again on audiogon and get reviews by people on ac      see my other ideas about that a few pages back      how about getting buyers to post here????????????????     maybe you need a course in sales and marketing because it looks like you are not trying to hard to sell or just going to the wrong places    have you thought of asking krell or bryston or somebody like that to put their name on your product or your guts in theirs??????????

i noticed you did post about the link i sent, but noone commented after that ............
guess that ends the discussion about amps and mechanical vibration !!!!!
about the topic of where to sell how many amps have you sold on amazon????

[art]

   have you thought of asking krell or bryston or somebody like that to put their name on your product or your guts in theirs??????????

I have been trying to stay out of this, for reasons that ought to be obvious, but this one has been a burr under my saddle long enough.

Hey, bub.............why on earth do you think that those guys who even consider that option? In effect, they would be admitting that everything that they have built in the past is doo-doo, and that they can't even do their own design work.

In an industry that is driven by the personalities of the faces of the front men for these companies, your suggestion is never going to happen. If you think otherwise, then we live in different universes.

OK, y'all can go back to pickin' on this other guy. I'll just sit back and watch some more. It is mildly entertaining.

Pat