Curious about the ADL600

Anyone know anything about this preamp? I just got to use one last week and it sounded amazing (IMO).

I did a search here and the only thread that came up was this one: bipolar tube supply. The block diagram suggests dual-triode opamp-like gain stages running on +/- 320 supply rails. Are these actual tube based opamp stages (eg Fred Forsell's tube opamp )? Google turns up a few interesting tube opamp links but only a couple are for bipolar supplies.

-fernando

As far as I could tell from prowling around in the case, it is indeed a tubed opamp. Nice sound all around, although I did find myself thinking, after the review was written, that I wouldn't mind hearing what it sounded like with Jensen transformers.

One thing to note: early runs of the ADL 600 had a manufacturing error that changed the input impedance of the mic pre section; the actual impedances came nowhere near the markings on the switch. Apparently there is a resistor on the board that is there for testing, but was supposed to get removed before the pre was shipped; in the early batches they didn't remove it. If you have one of those early units, Presonus will give you the information which resistors (since it's a stereo pre) to snip out.

Peace,
Paul

Thank you Paul.

Ignoring cost disadvantages for the moment, are there actual technical/sonic advantages to this topology (tube opamps and HV bi-polar supplies)...or is this just a case of deliberate over-engineering?

I reviewed the ADL600 for a German magazine a while ago. I found it a very interesting piece of gear, and given the high quality manufacturing, it is certainly not too expensive. The ADL600 is very clean. If you want something that sounds very tubey, you might be disappointed. Think of it as a clean preamp that does not sound like a transistor preamp.

Yeah, can't go wrong with Jensens (I own an A-Designs MP-2 :grin: ), but the Cinemags in the ADL aren't bad either. I personally could live without the impedance switch. If you're really exact about matching your levels, you'll find that the impedance switch doesn't do much, soundwise, in most cases. And whatever it does do in some cases is either random or a simple low cut.

[quote author="Rossi"]I reviewed the ADL600 for a German magazine a while ago. I found it a very interesting piece of gear, and given the high quality manufacturing, it is certainly not too expensive. The ADL600 is very clean. If you want something that sounds very tubey, you might be disappointed. Think of it as a clean preamp that does not sound like a transistor preamp.
[/quote]

Thanks Rossi. I did read your post about it.

It is superclean and three dimensional. IMO, it sounds subjectively cleaner than the clean pres I'm used to using (Grace, True Systems, Millennia, Hardy, etc.). The only tube pres I have used have definitely been on the coloured side, but I do know that "tube == coloured" is just marketing hype, but I was a still a little surprised.

I don't own one, but I've had a chance to listen, and in my opinion it's a very nice sounding preamp.

What I think is really great is Rossi's description of the impedance switch's action. This is my own feeling. Once you level match things, it's mostly either a low cut or more distortion. Either can work for a track, but just as often for me, I leave it at the "normal" position.

This is something true about all preamps, as far as I'm concerned, my own fully included.

In contrast, if you switch impedance via coil taps on something antique feeding a grid with no secondary load resistor, like a Collins 6P or 6Q, RCA BA-whatever, or anything with a UTC A-10, etc etc, you can get a radical eq change out of the mic (in the case of going too low on purpose) or a much more accurate and solid sounding match (50 ohm mics into 50 ohm coil tap).

Thanks Dan, you're bringing something up I didn't mention: distortion. Last year I wrote an article on variable input impedance, including frequency response measurements. When the article appeared, I got an e-mail from Martin Schneider, one of the chief Neumann engineers. In essence, he agreed with what I had written and added a note on measurements he had conducted in the area of distortion.

It's logical, when you think about it, that a mic output stage produces more distortion when it's loaded down severely. Depending on how the output stage is constructed those distortions may set in at medium levels. Other output stages (such as the TLM103's) are fairly resilient. Nonetheless, he thought it would be a good idea to remind people that a mic's specs - in this case max SPL - are only guaranteed at the given recomended/rated load impedance, usually at least 1000 ohms. I've yet to find a mic manufacturer that endorses low input impedances.

emrr, you're probably right about those old preamps. But I think modern day manufacturers make sure their preamps don't sound vastly different depending on the mic's output impedance (which, as we know, may vary from about 30 to 600 ohms on typical mics).

:thumb: :thumb:

I agree wholeheartedly with this characterization of input impedance. My suspicion with some adjustable or switched preamps is that it can just sound different which if it cost enough may be perceived as good.

Low impedance is useful for 50 ohm mics which AFAIK were used for film or location recording where the mic had to drive long lines. Nowadays such applications are covered with wireless mics so this may be obsolete or on the way.

The direction I am curious about is going much higher than the nominal 2k bridging termination. Every transformerless pre I ever did involved artificially adding resistance to terminate the input. While the 6.8k phantom build out may set a practical upper limit if useful, phantom power could be supplied via current sources or from a higher voltage to increase that impedance but there may be some operating point issues.

Rossi, in your communication with Neumann did they address input terminations >2K?

JR

[quote author="JohnRoberts"]The direction I am curious about is going much higher than the nominal 2k bridging termination. Every transformerless pre I ever did involved artificially adding resistance to terminate the input.[/quote]
FWIW the SSL 9k is an example of a micpre with a switchable Hi-Z input*, as I understood for making it line-in as well (together with engaging the 20 dB pad).
But people will perhaps be using it with mics as well. Maybe someone has some user-experience here ?

*: well, 'Hi', all is relative here, IIRIC somewhere around 10kOhm

My own experiments in microphone loading, done in a class I teach at the university, have had the following results:

SM57s and their relatives are greatly improved by a load impedance of about 500 ohms, achieved either by an alternate tap on the preamp's input transformer or a load resistor added in parallel with the standard tap.

On the other dynamic mics we tried (Electro-Voice PL20, Sennheiser MD421 mk I, Electro-Voice RE15) a 500 ohm load had audible effects that were barely detectable, and the listeners were split on whether they were an improvement or not.

The one condenser mic we tried with a 500 ohm load, a Shure SM81, sounded distinctly worse.

All these results make sense, if we assume that earlier mics like the SM57 family were designed with minimal mechanical damping, relying instead on electrical damping from a low load impedance, while recent dynamic mics were designed with significant mechanical damping, making electrical damping superfluous. The SM81's response to a lower-impedance load also makes sense, in that it's being required to source more current than it was designed for.

So far we haven't tried any transformerless-output condensers; I would expect them to also sound poorer with lower-impedance loading, but no data as yet.

As for higher-impedance loading: my speculation is that it would marginally benefit transformerless condenser mics (less distortion), but it might be a problem for transformer-coupled condensers, which could develop a peaky high end and ringing because of the inadequate electrical damping of the transformer.

Incidentally, while reviewing the ADL 600, I tried switching the load on a Beyer M260 ribbon microphone. The "600 ohm" position (actually lower due to the manufacturing error described above) sounded significantly better on mandolin, which is a real torture test on the high end. That surprised me.

Peace,
Paul

Since the defacto 1.5k-2k termination is pretty common it seems those dynamic mic manufacturers could easily add some resistive damping. They probably wouldn't if it also reduced sensitivity since at POS louder is usually better to general purpose mic customers.

Thanks for feedback.

JR

A tidbit: the mic mixers manufactured by Shure around the time of the 57's introduction had an input Z of ~800 ohms (xfmr coupled). It's reasonable to postulate that they may have designed the mic specifically to work well with their mixers.

[quote author="JohnRoberts"]
Rossi, in your communication with Neumann did they address input terminations >2K?
JR[/quote]

Yes, but not in great detail. You know, mic manufacturers have a different perspective. To them a preamp is not something they design or modify but a set of variables they have to take into account: input impedance somewhere from about 500 ohms about 10k, insufficient phantom powering etc. Variable impedance on modern preamps makes things even more complicated; some preamps go as low as 150 ohms.

There is some info on that in an AES Lecture by Martin Schneider "Electromagnetic Interference, Microphones and Cables" which you can download at the Neumann website (The 100K measurements there are actually about 13.6K because of the phantom power resistors).

On transformerless condensers, higher impedance often gives better bass response, simply due to the fact that the output capacitors form a hi pass with the preamps input impedance. The lower the impedance, the higher the cutoff frequency. Plus the effects on distortion charateristics.

I don't think it's necessary to go above 5 or 10K, or anything that's practicable with usual phantom powering concepts. I wouldn't expect any audible improvements above that. Except maybe for ribbons (see below) which don't require phantom powering, so you could disconnect both phantom power resistors for passive mics and have a very high input impedance for passive mics, if you want.

Paul, it's interesting what you say about the SM57. Incidentally I didn't conduct any tests with a Shure dynamic. The ones I used were Beyers which, like most modern day moving coil dynamics don't use an internal transformer. With ribbons, I found high impedance settings better than lower impedance. Apparently I'm not the only one. Both True Systems and AEA make dedicated ribbon preamps that have higher input impedance than usual, and I think Grace Designs have a model that sports a ribbon switch which increases the input impedance. On most ribbons, higher input impedance gives better treble response, and almost all give better bass response on high impedance inputs.

Very cool... My dad was a recording engineer for RCA in NYC in '55 they probably knew each other. The audio world wasn't that large back then. It's not that large now.

JR

Thanks Dave, that's a very interesting article. I actually got it in the said e-mail exchange with Neumann. I wish I'd had it, when I wrote my article. The BBC ribbon monograph has some interesting info, too.

The question of actual input load versus described input load is always another muddling factor.

I find my Beyer M160 / M130 ribbons don't sound all that great with my Sytek transformerless preamp (one would assume modern 1.5-2K input; not quoted), and sound much better with any vintage transformer coupled inputs driving tube grids. All my vintage transformer inputs drive grids from secondaries with no load resistors, and only refer to intended input impedances; 30, 50, 150, 200, 250, 500, 600, etc etc. The AC loads presented to the mics are not really specified or known. There are definitely sound differences when comparing the extremes to the middle, beyond the level differences caused by the varying ratios. The DC resistive differences of the coils versus a transformerless input must play a factor in damping, etc etc. And which preamp really has the higher input impedance?

End of muddled morning thoughts....

The first time I heard an ADL600 I felt like I should get one.  I finally did.

I've got a desire for a pultec EQ, and thought about using the ADL600 line input as the "make-up" stage.  While looking at the ADL600 specs a couple of things looked "odd" to me.

Referencing the manual:

http://www.presonus.com/uploads/products/2064/downloads/ADL600_OwnersManual_EN2.pdf

Section 3.2 says the line input has an impedance of 2k and the instrument input has an impedance of 100k.  Those seem off by a factor of 10 to me. 

Any thoughts on these specs?  I've only used an active instrument with the ADL so I didn't get a sense that it's impedance was really low.  I haven't used the line input either.

In the meantime, I'll fire off an inquiry to Presonus and see what they have to say.

fmed said:

Thank you Paul.

Ignoring cost disadvantages for the moment, are there actual technical/sonic advantages to this topology (tube opamps and HV bi-polar supplies)...or is this just a case of deliberate over-engineering?

Click to expand...

Without looking at the schemo, my basic view is that the advantages are well-known:
- an opamp-based structure has low distortion, high bandwidth and low output Z, which is useful when driving an output xfmr
- a bipolar supply allows using direct-coupling to the output xfmr, which eliminates the low-frequency hump that is inevitable with capacitor-coupling. Now have Presonus/DiMaria really implemented direct-coupling? I don't know but I would expect it.

The cost penalty of bipolar supply may not be great; it depends if they use regulated supplies or not.
Overall, the greates cost factor is the high-voltage rails. 600V means that the stages must be of a specific design (unless exotic tubes are used) such as totem-pole or other voltage-sharing technique.