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Old 8th October 2012, 10:59 PM   #51
boris81 is offline boris81  United States
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I'm sorry to hijack the thread but I need a quick advice and you guys seem very knowledgeable on the subject. I just bought a miniDSP and while evaluating it I discovered that it's very sensitive to different power supplies. (Contrary to the manifacturer's claim)
I'm currently using it with this one but I find the noise floor to be a bit high and the bass transients are still garbled. Can you guys point me to commercial solutions that could improve the miniDSP performance or do you consider it a bad design that can't be improved with a power supply?

What are desirable characteristics for a good power supply?
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Old 9th October 2012, 01:04 AM   #52
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Hi Boris, I'd say your core question would be best asked in the miniDSP forum; more likely that other users with the same issue will see it, plus miniDSP support may respond.

Posts 46 to 50 discuss one method of answering your secondary question. If you can figure out how miniDSP implements the supply topology on their board and post a schematic, board layout pics, and measurements here that would be an quite interesting data point to bring up alongside Cirrus's and Focusrite's design approaches. Sorry if this sounds picky, but it's hard to say much that's meaningful without some understanding of the design context. And, well, post 51 doesn't indicate which miniDSP units are in use much less resolve the guesses earlier in this thread as to what parts miniDSP is using.

DACs generally have around 50dB PSRR typ so one tends to need to hold analog supply ripple below a couple hundred microvolts to expose the DAC's DNR as the noise floor at the low PSRR corner. That's doable with regular parts but wants some attention to detail to pull off as it's demanding of regulators' own PSRR and noise floors---if noise performance beyond the usual LM78xx, LM317, LM1086, and LM1117 parts is needed the ADP150 and 151 are good starting points. For switchmode wall warts you pretty much have to measure to find out the switching frequency and ripple and then design from there as exceedingly few of them are actually specified. If you control the part choice it's rather easier; some of the parts in National's SimpleSwitcher line deliver a few millivolts of ripple at 500kHz, which is both small and easily rejected by audio band filters. However, DAC current draws are generally low enough switchmode isn't particularly cost effective compared to linear when one's DIYing the supply.

As an aside, ESS doesn't spec their DACs' PSRR but Dustin's indicated here on DIYA he designed for performance at the expense of PSRR so it's likely their PSRR is noticeably lower. That's likely why their parts require low noise, high PSRR, cap swamped op amps as regulators to hit the datasheet specs.

Last edited by twest820; 9th October 2012 at 01:16 AM.
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Old 9th October 2012, 04:25 AM   #53
boris81 is offline boris81  United States
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Twest, thank you for the quick response. The topics you are discussing are beyond my level of understanding so I doubt I can contribute much. The guys at the miniDSP forum suggested I evaluate it using a battery and now I can confirm that it performs very well (subjectively). I think the measurements you mentioned in post# 42 are genuine but the trick is finding a power supply that performs similar to a battery.
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Old 9th October 2012, 04:29 AM   #54
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The link you gave was to a switching supply - they do tend to contribute high levels of common-mode noise which is very hard to filter out. The battery has zero common-mode noise as its isolated. In between these two would be a linear power supply, using an EI core transforrmer - this has better isolation than a toroidal transformer but much worse than a battery.
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Old 9th October 2012, 07:15 AM   #55
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Yeah, the 0dB CMRR of unbalanced interconnects is one of the home audio's industry's more notable disservices to its customers. All it takes to build a differential receiver is two extra resistors and, since the default resistor tolerance is 1% at the moment, that's 40dB CMRR. Susumu's RR series is nice---0.5% for at most 1 cent more per resistor than 1% and pretty low temp co---and it's tough to find better than the RGs if one needs more than the 46dB CMRR the 0.5% RRs provide.

Most times I run the maths 40dB is sufficient to render the likely range of ground bounce between components inaudible if the gain structure of the system is well configured. But the extra 6dB from the RRs is cheap insurance. At the other extreme a THX certified power amp (28dB gain required) with unbalanced interconnects amplifies any bounce on its ground relative to the preamp by 28dB. With a typical AWG 16 power cord and the relatively large quiescent dissipation to output signal power of a typical 100-200W solid state amp used at conversational listening levels it actually works out the ground bounce created across the power cord's resistance by the charging pulses through the amp's rectifier is amplified enough that it's borderline audible. The details depend on the amp's ground structure as well as what's going on with the preamp/DAC's own charging pulses and the bounce is often down around a millivolt where it's tricky to measure. (Lest folks think I'm off my rocker here's a basic run through of the math: output levels for typical listening tend to be a few hundred mV and the edge of audibility is nominally around 50dB below signal so, with the amp's 28dB gain, the ground voltage across an unbalanced interconnect needs to be tens of microvolts or less to be inaudible. AWG 16 wire is 13 milliohms per meter and power amps generally pull around an amp in normal operation so the topology is not set up for success; it's aimed more at tens of millivolts instead of the tens of microvolts needed.)

Not clear from the supply thread in the miniDSP forum if Boris has the balanced or unbalanced version of the 2x4, though. Boris, which version do you have?

If one's DC coupling a DSP and DAC this problem gets a little trickier as the digital source may be USB or 1394, either of which may bring more ground noise than a wired SPDIF connection to CD player or a Squeezebox would. The circuit topology doesn't change, though; one still has a source, a DSP/XO/EQ/DAC/preamp thingy, and a power amp. They all have their own power cords, so there are three different loops for ground currents to run around in. Since it's all low impedance and the ground net is more or less everywhere the voltages are low and it takes good (read more expensive than most DIY budgets) measurement gear to get measurements good enough to have clarity on what's going on. Subjectively I think I can sometimes hear a small difference with the laptop and Saffire plugged into different wall outlets on the same circuit versus the same power strip, which would imply a kind of disturbing amount of noise in the ground loop from the laptop supply, but I've never been able to get a statistically signifcant ABX result out of it.
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Old 9th October 2012, 08:16 AM   #56
qusp is offline qusp  Australia
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right, so its changed from a digital input, digital output DSP/XO thread, to being hijacked as an all in one analogue out digital XO thread, to a mini DSP power supply support thread?

sorry I just find that really frustrating! I was really happy to see this thread started because I was going to open one of my own on the same topic, then I got a bit pissed when it was taken over by people claiming the single interconnect for clock meant the thing was unworkable, thus better off building or buying an entire design I dont need, which was the entire point of wanting to start the thread about not doing that. NOT having to buy and bypass the dac and analogue stages was the entire point and nobody will convince me otherwise, now its gone somewhat further back on topic, but not really.

so do I start another thread called high end ALL DIGITAL DSP crossover, no DACs allowed?

Last edited by qusp; 9th October 2012 at 08:23 AM.
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Old 9th October 2012, 08:40 AM   #57
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I agree, this was not what I expected when the thread was posted.
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Old 9th October 2012, 08:44 AM   #58
ChrisPa is offline ChrisPa  United Kingdom
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Quote:
Originally Posted by qusp View Post
right, so its changed from a digital input, digital output DSP/XO thread, to being hijacked as an all in one analogue out digital XO thread, to a mini DSP power supply support thread?

sorry I just find that really frustrating! I was really happy to see this thread started because I was going to open one of my own on the same topic, then I got a bit pissed when it was taken over by people claiming the single interconnect for clock meant the thing was unworkable, thus better off building or buying an entire design I dont need, which was the entire point of wanting to start the thread about not doing that. NOT having the dac and analogue was the entire point and nobody will convince me otherwise, now its gone somewhat further back on topic, but not really.

so do I start another thread called high end ALL DIGITAL DSP crossover, no DACs allowed?
I thought it had been hijacked until I read twest's last post, which I found completely on topic

A 'best' DSP however it is implemented and whether all digital or whether it contains analogue interfacing, is dependent on its interconnection and interaction with upstream and downstream components, and the quality of its supporting circuitry and detailed implementation (stating the obvious)

Back to the OP. Some of the respondents here consider 'state of the art' and 'all digital' not to be completely compatible, and it's therefore a fair response if the answers err away from 'all digital' and they explain why

Similarly, the answers so far would indicate that no-one is building a 'state of the art all digital' - so does that mean the thread is or should be closed?

Instead we have seen references to the closest anyone seems to be doing to the OP's spec whether or not they are 'all digital' and whether or not they actually exist yet - that seems fine to me too. And since such a device doesn't seem to exist, then observations on implementation - eg. balanced connections and psu considerations also seem relevant to me

Anyway, let's allow any relevant/on-topic posts to continue
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Old 9th October 2012, 09:11 AM   #59
ds23man is offline ds23man  Netherlands
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This thread has been of topic from the beginning, it has been only about how to transport the digital stream to and from the DSP board.

But the main question is: what is a state of the art DSP?

So we have to get some answers on the next questions:

1. What is the best DSP chip out there?
2. What is the influence of the software running the chip?

I/O problems can be solved, but how do we "define" the state of the art DSP!
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Old 9th October 2012, 09:19 AM   #60
qusp is offline qusp  Australia
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ChrisPa, no respondents vs hijacked from the beginning are 2 completely different things

twests last post was somewhat more on topic, but still not really on topic, digital in-out noise floor and psu quality for that is pretty different to noise floor and gain structure in the analogue stages are they not? the analogue noise floor being carried through from the digital source and whether to isolate or not is relevant, but I could just see the next post being another mini DSP support question. just because there are few products in this category doesnt mean there shouldnt be a thread, this is DIYAUDIO isnt it?

i'm just cooking dinner, so will post something a bit more relevant in the next couple hours.

Last edited by qusp; 9th October 2012 at 09:27 AM.
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