Purity's "high-resolution 200-watt switching amplifier"?

The Martin Logan Purity line included "internally powered, high-resolution 200-watt switching amplifier." Neither their website or literature provides much detail regarding these amplifiers.

Supposedly, "to achieve ultimate performance with the Purity use of the RCA line level connection is recommended. With this input Purity is able to connect directly to devices with variable level control such as digital music players, flat screen televisions, computers, and receivers with pre-amp outputs." Further, under Specs/Impedance it shows: Binding posts ~ 2,000 Ohms; Line level RCA ~ 14,000 Ohms. Compatible with 4, 6, or 8 Ohm rated amplifiers.

Question: Has anyone received additional details from ML regarding these internal amplifiers?

So I guess I'm confused. I assumed one should connect an external 4, 6, or 8 Ohm rated amplifiers to the binding posts, and that the line-level connector was to be used exclusively for digital signal input. Isn't the purpose of the "high-resolution 200-watt switching amplifier" to convert the digital signal to analog before energizing the electrostatics and bass speakers?

Do the Purity amplifiers meet ML's standard? "...the amplifier [should] be stable operating into varying impedance loads: an ideally stable amplifier will typically be able to deliver nearly twice its rated 8 Ohm wattage into 4 Ohms and should again increase into 2 Ohms?"

Welcome Reality

There's no DAC inside the Purity. It takes line-level analog audio input from a pre-amp output (or MP3 player, etc). It's like a powered subwoofer... but it's powered for the entire frequency range. Optionally, you can drive the Purity's binding posts with a standard amplifier, but the relatively high input impedance (2,000 Ohms) suggests there's a voltage divider or similar attenuator to feed the signal into the internal switching amp for RE-amplification to the driver elements. I suspect this is one reason why they recommend using RCA inputs for best results (i.e. avoid the re-amplification).

Note also that a "switching amplifier" is just a nother type of analog amplifier... there are several types including Class A, B, A/B... and then "D" which are sometimes refered to as switching amplifiers. While some "d" amps use a Pulse-Width modulation scheme, it has nothing to do with digital autio formats such as SPDIF. None of these analog amplifiers include digital to analog conversion. That's not to say they don't exist - I think Meridian might have some sort of hybrid internal amp with a D/A front-end... but definitely not Martin logan.

Cheers

Thanks! tsv_1

I just plugged my iPhone 4S into the line-level connectors using a short cheap cable and I must say the sound is rather good (streaming MOG @ 320kbps over 20Mbps Internet connection), comparatively speaking to my ML Motion 4 and sub-woofer, despite what many consider the "cheap dac" contained in the iPhone. Since the iPhone makes a killer remote (Pandora, MOG, etc.), I'd like to get some higher-quality receiver/dac/transport/etc, use the pre-amp output, and quality cables to improve the sound quality further. Any suggestions?

RealityBroker said:

Thanks! tsv_1

I just plugged my iPhone 4S into the line-level connectors using a short cheap cable and I must say the sound is rather good, comparatively speaking, despite what many consider the "cheap dac" contained in the iPhone. Since the iPhone makes a killer remote (Pandora, MOG, etc.), I'd like to get some higher-quality receiver/dac/transport/etc, use the pre-amp output, and quality cables to improve the sound quality further. Any suggestions?

Click to expand...

It really all depends on what you use your system for. Are the Puritys part of a larger multi-channel HT setup? Or just 2-channel + video? Or just 2-channel audio? Your options range from a small external DAC like one from NuForce - to a decent receiver with pre-outs - to a full blown system of separates. Or keep things just the way you have them now, download "My Tunes" from the apple app store and mess around with the settings... you might find some interesting results just spending $4.99 on that.

I know there are several people here that are big on streaming, servers, software, etc as well... tons of options there too.

Cheers

It's just a little B&O Icepower amp module, as used in all the ML powered speakers, probably a 200ASC or similar. Its nothing too special.

tsv_1 said:

None of these analog amplifiers include digital to analog conversion. That's not to say they don't exist - I think Meridian might have some sort of hybrid internal amp with a D/A front-end...

Click to expand...

Technically, a D/A back end Todd (amplification before D/A conversion), and they're often characterized as a "Power DAC". Haven't heard that Meridian makes one, but TacT, Behold, NAD (M2) and a very few Pio/Sony AVR's were/are made using TI's Equibit (née Toccatta) technology.

RUR said:

Technically, a D/A back end Todd (amplification before D/A conversion), and they're often characterized as a "Power DAC". Haven't heard that Meridian makes one, but TacT, Behold, NAD (M2) and a very few Pio/Sony AVR's were/are made using TI's Equibit (née Toccatta) technology.

Click to expand...

Back-end? That does not compute (monotone computer voice). Can you send me a link or pointer to a write-up on any implementation that is based on power amplification prior to D/A conversion? I suppose you could say that the Equibit is a DAC topology that outputs the analog waveform in an amplified fashion... but the high slew rates required for an implementation where the digital input stream (or bus) is amplified before PWM occurs just isn't practical (think of the emi implications).

Here's a blurb from Equibit's own literature (I looked it up because I just couldn't believe the amplification occured prior to D/A conversion):

1 Introduction
The TAS5000 is an innovative, cost-effective, high-performance 24-bit stereo digital modulator based on Equibit
technology. This product converts input PCM serial digital audio data to an output PWM audio data stream. The
TAS5000 is designed to be connected to two TAS5100 mono true digital amplifiers for driving loudspeakers. This
all-digital audio system contains only two analog components in the signal chain—an L-C low-pass filter at the
speaker terminals. It can provide up to 90 dB SNR at the speaker terminals.


and this from Tact's literature:

Once the decision of the duration of the pulse is made the central processor controls
FET-switches at the output with extreme precision. Voltage and current are
drawn from the power supply and fed to the speakers.

tsv_1 said:

Back-end? That does not compute (monotone computer voice). Can you send me a link or pointer to a write-up on any implementation that is based on power amplification prior to D/A conversion?.

Click to expand...

Todd, the best paper I have shows the circuit topology, but is no longer available via link without payment. If you'll PM me your email, I'll be happy to send you the .pdf.

Closest thing I've got, aside from the .pdf, are quasi-vague descriptions from reviews:

TacT Millenium
NAD M2
Behold

RUR said:

Todd, the best paper I have shows the circuit topology, but is no longer available via link without payment. If you'll PM me your email, I'll be happy to send you the .pdf.

Closest thing I've got, aside from the .pdf, are quasi-vague descriptions from reviews:

TacT Millenium
NAD M2
Behold

Click to expand...

Not that I'm into necro-equine harrasment (that I'll admit to )... but even in the first review I clicked on, they clearly state D/A happening prior to final amplification:

" For instance, in a full Behold system (sans loudspeakers and speaker cables), the digital-to-analog conversion is performed not in the source component, or even in the processing unit or separate DAC, but in the power amplifier -- the last stage before the signal is amplified and sent on to the speakers."

Thanks for the offer to send the pdf... perhaps someone has come up with a method of pre-amplified D/A... in which case, I look forward to learning something new.

Cheers

OK... I think I see where the disparity is (i.e. disparity in terminalogy that is)... the following statement from the pdf (thanks for sending BTW):

"After the power stage, the last step of the audio signal pathway is through a low-pass LC filter, which removes
high-frequency components from the audio signal to reduce EMI. This is effectively the digital to analog conversion.
Without this inductor and capacitor filter, the speaker itself becomes the digital to analog converter (DAC)."

The key word here (as I see it) being "effectively"... the author suggests that the filtering effects of the low pass LC
network basically complete the conversion process. Fair enough. While I would argue that the actual conversion of PCM to PWM
is the heart of the D/A process, I won't argue that without final filtering the process isn't really complete.

Cheers

Todd, I think the following from the paper best describes the important difference:

"The conversion from digital to analog occurs at the final output voltage, avoiding unnecessary noise that will be audible. In contrast, the analog conversion occurs as the first step in an analog amplifier and is at a low level voltage, which is then processed and amplified. Any noise on this lowlevel signal from processing or from coupling will be amplified and affect the final signal quality."

I own a number of amps, including two TacTs. I'll avoid the usual subjective hyperbole and simply say that one must hear one of these "pure digital" amps in order to appreciate the differences. Unfortunately, they don't like Martin Logans. :-(

Abject apologies to the OP for ongoing OT amp discussion.

No apology necessary. That's how we learn. I'll simply read and re-read until the fog dissipates.

RealityBroker said:

No apology necessary. That's how we learn. I'll simply read and re-read until the fog dissipates.

Click to expand...

Thanks Reality for your tolerance.

Ken... I didn't know about the sensitivity of this kind of amplifier to MLs - can you elaborate? Is it the low Z at HF that causes trouble?

tsv_1 said:

Ken... I didn't know about the sensitivity of this kind of amplifier to MLs - can you elaborate? Is it the low Z at HF that causes trouble?

Click to expand...

Doesn't appear to be low Z, Todd. Worked flawlessly for a few months and then failed on turn-on, and failed again on turn-on after each of two repairs. Bought another TacT amp on the cheap via A-gon and ditto. Both manufacturers tried their best, but both claim no fault on their end. Boz (TacT's designer/owner) ultimately felt pretty sure he could add a diode at the input and preclude any evil turn-on artifacts from the ML's, but I took a left at Albuquerque (Sanders ESL) to escape the problems and so haven't taken him up on his offer. Yet. ;-)

RUR said:

Doesn't appear to be low Z, Todd. Worked flawlessly for a few months and then failed on turn-on, and failed again on turn-on after each of two repairs. Bought another TacT amp on the cheap via A-gon and ditto. Both manufacturers tried their best, but both claim no fault on their end. Boz (TacT's designer/owner) ultimately felt pretty sure he could add a diode at the input and preclude any evil turn-on artifacts from the ML's, but I took a left at Albuquerque (Sanders ESL) to escape the problems and so haven't taken him up on his offer. Yet. ;-)

Click to expand...

Any explanation that includes a Bugs Bunny reference is good enough for me Sorry to hear you had so much trouble with those amps.

Back to Reality(broker)

For now, 2.1 or 5.1 channel audio. So I'm researching the various AV Receivers, with pre-amp outs. Also, my wife and I like using our iPhones as remotes, so a proprietary app or AirPlay is necessary. I am curious why ML would state "To achieve ultimate performance with the Purity use of the RCA line level connection is recommended" if as unclemonty claims the internal amplifiers are "just a little B&O Icepower...200ASC or similar...nothing too special." Nevertheless, with a full-bells-and-whistles quality receiver, I would always have the option of simply using the binding posts and ignoring the internal amp.

RealityBroker said:

For now, 2.1 or 5.1 channel audio. So I'm researching the various AV Receivers, with pre-amp outs. Also, my wife and I like using our iPhones as remotes, so a proprietary app or AirPlay is necessary. I am curious why ML would state "To achieve ultimate performance with the Purity use of the RCA line level connection is recommended" if as unclemonty claims the internal amplifiers are "just a little B&O Icepower...200ASC or similar...nothing too special." Nevertheless, with a full-bells-and-whistles quality receiver, I would always have the option of simply using the binding posts and ignoring the internal amp.

Click to expand...

If by "ignoring the internal amp" you mean bypassing it, I don't think there's a way to easily bypass it. The binding posts are simply a convenience for speaker cable connection. You would still be driving the internal amp from your receiver's amp (thus my comment earlier re: "re-amplification"). Perhaps the internal configuration of amp and crossover can be hacked into to allow driving the transducers directly from an external amp... but that would require sleeve-rolling, schematic interpreting, and solder smoldering.

Again, I think ML is suggesting best performance with the RCA input because it's likely the cleanest/shortest signal path (i.e. not cascading amplifiers). This is my take on it anyway.

You're best bet would be to do just what you're doing (i.e. research receivers with well reviewed pre-outs).

For what it's worth...

When I had Puritys (Purities?) I tried them both ways. I thought they sounded better direct from the pre-amp than with an amp (Carver tfm-45) in the path.

mandd