New miniDSP 2x8 / 4x10 card

JohnR · « **Reply #100 on:** 7 Sep 2011, 03:22 pm »

Um. Yeah OK, so you won't take Dave up on his offer?

pelliott321 · « **Reply #101 on:** 8 Sep 2011, 07:46 pm »

Its not that I am not going to take Dave up on his offer, which I appreciate. But what can he do DSP has measured the unit and stated it is in spec and that there is out of band noise which must be inherent in the design. So all units have this. Now when it comes back I will put it back on the bench and try to filter out this noise using the scope. If I can get rid of the noise and it sounds better then all is well

Davey · « **Reply #102 on:** 11 Sep 2011, 01:55 pm »

Something still doesn't add up. I have a hard time believing 0.5 volts of noise...in or out of band...is normal operation. Maybe you misunderstood their explanation?

However, you seem determined to declare your unit is not malfunctioning, and you won't let someone with some proper test equipment give you a second opinion?

Interesting.

Dave.

JohnR · « **Reply #103 on:** 12 Sep 2011, 01:03 pm »

Quote from: pelliott321 on 8 Sep 2011, 07:46 pm

But what can he do

Hi pelliot, he could provide an independent assessment with meaningful analysis. In particular, the actual spectral analysis. Having been out of hardware and in software for the last two decades. I'm afraid that my own test bench is fairly miserable, but Dave's is not and since he's offered, it seems like it would just be helpful to do it that way. But it's your unit, of course.

JohnR · « **Reply #104 on:** 12 Sep 2011, 01:16 pm »

Sorry... I just feel that i need to counter some of the negativity here with an alternative viewpoint. I had somebody come over to my house last week to listen to my system based (now) on the 2x8. Written comments included "John's experiment with Dipole OB drivers and miniDSP is impressive" and "sounds bloody brilliant". Not tooting my horn, I'm well aware of the limitations of my system at present, but it's a counterpoint showing what can be done.

He did in fairness also say "The only criticism I have is some sibilance, which could be from the electronics or from his $60 ribbons" which is an issue that has been commented on every iteration for a year now, and which I attribute to those "$60 ribbons" and I'd be happy to be proven wrong if anyone cares to send me a pair of $500 ribbons!

pelliott321 · « **Reply #105 on:** 12 Sep 2011, 01:46 pm »

I do not understand why I am getting beat up over this. I chose to take the time and spend for the shipping to send it back to DSP to save time if the unit was bad. They would send me a new one and that would be that.
I will be glad to send this unit over to Daves for a look see. You can e-mqil your postal address to me at pelliott3212Yahoo.com.
I do not have $60 ribbons. I have rebuilt Maggy IIIa's with factory rebuilt Ribbons (new a year ago) which are the same Ribbons that are used in all the new 3 and 20 series. The IIIa's are supplemented with a pair of Janis Wi 15 inch subs presently using a Marchand electronic xover
I use a Sota Star Sapphire TT with Shelter 501 MC, Vandetta Research step up to a Transcendent Sound Phono and Grounded Grid preamp. On the Digital side I use a Twisted Pear Buffalo II DAC from my music server.
I am proud of my system and I feel it sounds very good. This try of going fully active is just an experiment, nothing more.
I do not like the assumption that I am in the wrong here just because I questioned the quality of sound of this unit.

Davey · « **Reply #106 on:** 13 Sep 2011, 04:10 pm »

Nobody's questioning your evaluation of the sound of the unit. We are just trying to determine what the "issue" is.

Since the scope photo's you posted raise question marks we're all trying to understand. Maybe this is some sort of operational issue the fellas at miniDSP couldn't duplicate? Or maybe some sort of equipment interfacing problem? I dunno, but I'm (like a few other folks) curious.

Email with address is on the way.

Cheers,

Dave.

pelliott321 · « **Reply #107 on:** 15 Sep 2011, 06:51 pm »

I just got the unit back and I will give it a try and see if I can duplicate what I heard and saw on the scope before.
Davey: I got your email and will contact you via email
I apologize for not responding more quickly but I do not live on the web.

pelliott321 · « **Reply #108 on:** 16 Sep 2011, 12:14 pm »

here are some new (better) scope shots
this is with no input on my mid channel (its the same on all channels) You are looking at the B channel of the scope and the setting are V/D is .2 Sweep is .1ms

The bottom trace is input from sig gen. Top trace is output of mid channel show noise embedded in the sine wave

JohnR · « **Reply #109 on:** 16 Sep 2011, 12:33 pm »

There's really not much can be read from those.

jtwrace · « **Reply #110 on:** 16 Sep 2011, 01:05 pm »

I'd like to suggest rather then "dog" the unit, just send it to Dave who has graciously offered to test it with the proper equipment / experience.

pelliott321 · « **Reply #111 on:** 16 Sep 2011, 04:44 pm »

Dave sent me his address and I will try to get it into the mail on Saturday, if not definitely Monday

JohnR · « **Reply #112 on:** 21 Sep 2011, 09:10 am »

Cool - I and I'm sure others will be interested to read his analysis.

Davey · « **Reply #113 on:** 25 Sep 2011, 04:13 pm »

I received Paul's miniDSP 2x8 board and also checked another one. Both measure the same so I think we can assume this characteristic is normal.

There is a decent amount of high frequency noise on all the outputs........even relative to the output noise from the miniDSP 2x4 boards, which is somewhat higher than analog circuitry would create. I have a theory on the cause of this, but it's speculative.

Anyways, here's a couple of photos from the many I captured.
The first one is a 1khz sine input/output of 200mV RMS for both the 2x8 board and 2x4 board. (200mV would be typical value these boards might see for normal operation.)
The second photo shows noise floor (no input signal) of the 2x8 output (blue) and 2x4 output (red) over a range of 0-300khz. In the audio band the noise floors are similar, but it's easy to see the large increase in output noise above 50khz generated by the 2x8 board.

Cheers,

Dave.

pelliott321 · « **Reply #114 on:** 26 Sep 2011, 11:57 am »

Thanks Dave for your time and effort and validating what I found.
The whole idea of this board and the very easy to use software is very good. But if my board is representative of miniDSP efforts, well that is not very good.
I have forwarded your results to miniDSP to see if they will respond.

JohnR · « **Reply #115 on:** 26 Sep 2011, 10:06 pm »

Dave, that seems to be about the frequency of the negative (I think) rail generator - ?

mgalusha · « **Reply #116 on:** 26 Sep 2011, 11:03 pm »

Kinda begs for a liner power supply in place of the on-board switchers. My board (2x8) arrived on Saturday though I haven't done anything except look at it.

Anyone know if schematics are available?

Davey · « **Reply #117 on:** 27 Sep 2011, 12:00 am »

This is not unlike the noise spectrum I've seen from various other wide-band gadgets on my test bench. In fact, I was looking at the output noise from my E-MU Tracker-Pre just the other day and it's very similar.

I guess it just highlights the difficulty in designing these sorts of gadgets at a price point with the almost ubiquitous use of switcher supplies nowadays.

It's difficult to judge how any of this translates to subjective audio performance because we don't know the characteristics of the downstream amplifier, the signal levels that will be used in the 2x8, etc, etc. However, it sure doesn't look good to an analog design guy like me.

Cheers,

Dave.

JohnR · « **Reply #118 on:** 27 Sep 2011, 12:07 am »

Dave - is the result the same with the analog inputs muted?

jtwrace · « **Reply #119 on:** 27 Sep 2011, 12:10 am »

Sorry if I've missed something. Davey-do you do this for a living?

Just curious.