Curious about the ADL600

Rossi said:

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.

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Agreed, altough you have probably, I'm sure, noticed that the effect is quite different on dynamic than on condenser (or any active) mics.
I have never noticed a significant difference on non-transformer condenser mics, a subtle one on those with a transformer, but on dynamics it's clearly audible. I use a lot the SM7B for male vocals and I wouldn't want it to see anything lower than 2kohm.
The preamps I use regularly that have this feature (ISA430MkII, LA610) end up being always in the highest impedance setting.

Quite an old thread revived!

Basically, impedance switches on newer preamp only give you the option to lower the input impedance, so I regard the highest setting the standard. Over the years, I conducted several tests and measurements with various moving coil dynamics, ribbons and condenser mics, and my conclusion was that they all sounded best in the highest impedance setting. That's what I meant, when I said I could live without an impedance switch.

The only exception to this rule seems to be the SM58/57. As Paul suggested, it profits from a lower input impedance of 500-600 ohms. Its design seems to rely on external loading. A couple of years ago, I built a little box with a passive filter that takes care of the upper midrange bump (same technique as in the Beyer M201). I prefer this to a low impedance input, because it produces an external load in exactly the right frequencies. The SM7B filters work in a similar way, too.

How do you like your LA610?  I own one, too, but I hardy ever use it. Always sounds kind of fuzzy to me.

Rossi said:

Quite an old thread revived!

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Indeed! Which proves that some subjects are never outdated.

Basically, impedance switches on newer preamp only give you the option to lower the input impedance, so I regard the highest setting the standard. Over the years, I conducted several tests and measurements with various moving coil dynamics, ribbons and condenser mics, and my conclusion was that they all sounded best in the highest impedance setting. That's what I meant, when I said I could live without an impedance switch.

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I agree with you; it just clutters the front panel!

The only exception to this rule seems to be the SM58/57. As Paul suggested, it profits from a lower input impedance of 500-600 ohms. Its design seems to rely on external loading. A couple of years ago, I built a little box with a passive filter that takes care of the upper midrange bump (same technique as in the Beyer M201).

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Yes, I remember that you posted a link to the article where you describe your box. Although I own a 75th anniversary 57/58 set, I never use them for recording so I never felt the need to investigate the effects of loading.

How do you like your LA610?  I own one, too, but I hardy ever use it. Always sounds kind of fuzzy to me.

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It's my go-to preamp now. I've read about the fuzzy/unclear criticism, but I never felt that. It may be something that they solved on the MkII.

So you have a LA-610 MK II? Mine is the original version. You know, I really want to like it, and when I look inside, I see good components. But I don't like the sound I'm getting. Perhaps I should give the MK II a try and sell my unit. I'm pretty sure you wouldn't like yours if it sounded like mine.

Back on topic: I don't think anybody has mentioned this yet, but Fred Forssell must have been involved in the ADL 600 design. The unit is listed among his design projects: http://www.forsselltech.com/#!/design

Makes me like the ADL 600 even better. 

Rossi said:

So you have a LA-610 MK II? Mine is the original version. You know, I really want to like it, and when I look inside, I see good components. But I don't like the sound I'm getting. Perhaps I should give the MK II a try and sell my unit. I'm pretty sure you wouldn't like yours if it sounded like mine. 

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After reading a number of comments on both units I started to think that maybe there was a serious design issue in the first version that has been solved in the second. It's a pity UA have never acknowledged that and never publish a modification.  Like if they want to show they never make mistakes. And I don't know anybody who has a reliable schemo of any of the versions.

Back on topic: I don't think anybody has mentioned this yet, but Fred Forssell must have been involved in the ADL 600 design. The unit is listed among his design projects: http://www.forsselltech.com/#!/design

Makes me like the ADL 600 even better. 

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Yes, he has a good list of credentials.

I ressurrected this thread because of the discussion about ADL600 and impedances.  It seemed appropriate.

It would appear that on the unit I have, the channel gains are off from one another (1db) and the meters seem to be a little off as well.  I've asked for a calibration guide and some clarification on the impedance specs.

Sadly, I've not heard back from Presonus.  I can't register the unit on their website as it doesn't seem to recognize the serial number.  I sat on the phone for an hour on hold and never got through to anyone.  I left a VM which generated a ticket on Friday...But no responses yet.  I'm pretty patient, but I would like to get the unit calibrated and ready to roll so I can stick it in a rack and get it patched in.

We'll see what happens.

I've been using the pre though and it's really quite a treat.  Very pleased to have it in my collection.

abbey road d enfer said:

After reading a number of comments on both units I started to think that maybe there was a serious design issue in the first version that has been solved in the second. It's a pity UA have never acknowledged that and never publish a modification.  Like if they want to show they never make mistakes. And I don't know anybody who has a reliable schemo of any of the versions.

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That's my impression, too. But it's hard to make any improvements without a schematic. Here's a guy who apparently reduced distortion by using ECC82 tubes on the output(s).
http://www.foxaudioresearch.ca/LA610.htm

To come back to the impedance issue: low input impedance does affect transformerless condenser mics, too. Just not at usual levels. Of course it depends on how the output stage is designed.

We tend to think of mics as low level sources, but modern condenser mics can produce around +10 dBu at maximum SPL. That's quite a lot, and it's quite a task to drive 1kOhms at such high levels at less than .5  THD - especially when you keep in mind that the mic has to do with a total current draw of about 5 mA (including input stage, DC converter and losses at various points). It becomes nearly impossible at loads much lower than 1 kOhms.

But then again, people may use the impedance switch to provoke distortion.

Rossi said:

Here's a guy who apparently reduced distortion by using ECC82 tubes on the output(s).
http://www.foxaudioresearch.ca/LA610.htm

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..and that is the MKII-model he fixes, not the even more unfortunate MKI...!!


As for large-outputs from condenser mikes on loud sources, I have often noticed significant improvement in quality by running them into a line input (i.e. for some of our non-phantomed mics, the U47, M269 and G7) in stead of using mic in.

The very good-sounding Calrec UA8000 mic pre switches to a separate "high-level", high impedance microphone input amplifier for gains <10dB - http://gyraf.dk/schematics/Calrec_UA8000_mic_input.gif

Jakob E.

gyraf said:

As for large-outputs from condenser mikes on loud sources, I have often noticed significant improvement in quality by running them into a line input (i.e. for some of our non-phantomed mics, the U47, M269 and G7) in stead of using mic in.

The very good-sounding Calrec UA8000 mic pre switches to a separate "high-level", high impedance microphone input amplifier for gains <10dB - http://gyraf.dk/schematics/Calrec_UA8000_mic_input.gif

Jakob E.

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A number of audio designers have pulled their hair over this subject; there are two confronting clans: 
Those who have tried to design THE ultimate universal mic pre, that caters for levels from a bee's fart to a nuclear Armageddon, and those who have taken the approach of optimizing their preamp for a more focused range of applications.
The Calred clearly seems to be some kind of hybrid, reminding me of the late Schlumberger desks, that had three different mic pres sharing a common transformer, switched according to the gain range, in order to present the user with a unique interface.

For a modern product, it seems rather academic now, considering that a THAT 1510 or a PGA2500 can deliver most of the goods (although they both need the addition of an input pad for super-high levels), but for a pseudo-vintage product, it's not an option.
For my part, in the cadre of music recording, I am an advocate of the "one tool for a task" concept.
Just like selecting the right mic, selecting the right preamp is part of the fun.

I have designed a 500/51-format mic pre that is specifically oriented to high level sources. It is based on a custom xfmr with two switchable primaries: a standard 200r-rated, and another rated at 20k, that is used to attenuate the signal by 20dB, while the secondary is 2k nominal. The actual resulting input impedance in the high-level position is about 8k.
I use them on drums, and although I can't claim it sounds significantly better than my other "ordinary" preamps, it definitely boosts my ego.  ???

I like your mic pre concept!

My complaint about the LA-610 is not that it doesn't cover all applications, but that I can't find any application that it really excels in. Short of a schematic, I think I'll give the ECC82 mod a chance, and if that doesn't do it for me, I'll just sell it.

I'm quite happy using different mic pres for different tasks. Apart from various solid state units, I own another tube preamp that I really like, the A-Designs MP-2. I thought the LA-610 would give me something similar plus some classic opto-compression, but I was wrong. That's too bad, because I think the hardware does have great potential.