LA-610 Mod

[emrr]

I agree that 5V heaters are not any indication of a problem, unless you can definitively show a measured improvement with it raised to the typical 6-6.6V range. 

I'd like someone also to measure the exact gain differences with tube type swaps.  In practice I imagine 1-2 dB, again negligible. 

As to practical reality, this is a repro of an american design from the time when all input was from low output dynamic mics.  If you use a modern high output dynamic, or a condenser of any type, and are unhappy with headroom, you are really using the wrong mic for the system as designed.  It does not imply there is a problem with the design, only the expectation.

 

[sr1200]

To add to emrr's post, the sm7b is probably the best sounding mic through this pre IMO, next to the lower output U87 clone, so there is definitely something to be said about that.  My Shure KSM's and other condensers tend to be brighter than those two (but not harsh like the original aricle suggests), but you really gotta be conservative with the input/output using those mics.  (if anyone saw my posts about the NTK ive been wrestling with, this was part of that problem apparently)

 

[mitsos]

Is the input trafo wired 1:14 or 1:7?  If 1:14, would rewiring it as 1:7 help?

 

[leadbreath]

most interesting...

 

[Rossi]

Well, this is not a true vintage reproduction but a fairly modern unit, loosely based on some older designs. It actually performs better with condenser mics than low output moving coil or ribbon mics. The problem is not with input overload but distortion within the input section rising to undesired levels due to low internal headroom.

In my listening tests today, I liked the unit a little better than before the heater mod. As I said, the distortion measurements didn't change much, so maybe that's a wrong impression, or maybe it's because I understand the unit better than before and work around the gain staging more wisely. The difference between measurements and subjective listening test may also have to do with the fact that I used the line input for measurements while I used the mic input for listening tests.

I still don't like the preamp section as much as my A-Designs MP-2. But the LA-610 sounds more usable now.

Thanks for the underheating info. From what I've read elsewhere, too low heater voltage can actually reduce tube life, depending on tube type. In any case 5.8V seems a more reasonable voltage than before. As I said, I don't intent to implement more changes. Without a schematic, modding would be mere guesswork, and it if I decide to sell the unit, any mods would make things more difficult.

By the way: If you have one of the older LA-610 units, measure the phantom voltage. Not just the voltage but also the maximum current.

 

[EEMO1]

Rossi,


ummm... could I bother you to measure some voltages from your unit? I think mine is not feeling too well...

 

[Rossi]

Sure, but it may take a while as the unit is already back in my rack and I'm in a hurry facing a deadline.

Also keep mind that the LA-610 went through a number of revisions. There seem to be at least two distinct variants of the old LA-610. Mine is the one with the 6072 output tube. If yours has a 12AT7 instead, it is a later one with a different circuit. Unfortunately not much else is known, and UA seems to have a no-schematics-to-the-public policy. So you'll like need UA for any kind of service except simple tube replacements.

 

[EEMO1]

The unit had 6072 installed when I got it.



thanks!

 

[emrr]

Well, this is not a true vintage reproduction but a fairly modern unit, loosely based on some older designs. It actually performs better with condenser mics than low output moving coil or ribbon mics. The problem is not with input overload but distortion within the input section rising to undesired levels due to low internal headroom.

Input transformer of similar ratio, same input tube type in appropriate circuit surroundings, likely close to the original with similar voltages; I'm not sure how it could be significantly different in gain architecture or headroom.  I don't imagine it to have less headroom than an original.  I see the original supposedly has an internal lo/hi gain switch on stage 1, and I have no idea where the repro lands in that regard. 

I'm not sure I follow you on 'low internal headroom' as distinct from overload point in regards to distortion.  Can you say it is any different from the average design of it's era?  I assume it's fairly similar to designs of that era, and therefore has to be used with the techniques of that era to achieve user satisfaction.  If designs of that era don't give user satisfaction, it's user error to continue using them. 

 

[Rossi]

The thing is, a hot condenser mic won't overload it (BTW. it wan't just ribbons back then, the good old U47 was pretty hot, too), but once you amplify the signals to nominal studio level (+4 dBu) there's not much headroom left before it goes nasty. Also, the distortion characteristic isn't particularly sweet. The spectrum is not just k2 and k3 but is pretty rich with higher harmonics.

I can't compare it to the original 610 console pres, obviously. What I can say, though, is that the Solo 610 I once reviewed sounded much better to me, I'm pretty sure it had more headroom, too. I have no headroom issues with my other tube preamp, an A-Designs MP-2, either.

I disagree that you should treat the LA-610 as a vintage unit. When you put out a new unit, even it it is based on older designs, you better make sure it performs well in today's environments. Nobody buys a unit because it would have been great to have 50 years ago; you buy it because you want to use it for the next 20 years or so. Also note that the LA-610 has some more "modern" ingredients such as P48, a DI input and switchable input impedance (the latter does not change the transformer ratio, it's probably just a loading resistor on the transformer secondary).

Anyway, I'm not so sure the LA-610 would have been popular back in the day. The real vintage stuff I know tends to be cleaner than today's retro units.

 

[sr1200]

And the reply from UA...

Thank you for contacting Universal Audio.

The heater voltage is 5V by design. There is no particular reason why they are at this voltage.

 

[Rossi]

And the reply from UA...

Thank you for contacting Universal Audio.

The heater voltage is 5V by design. There is no particular reason why they are at this voltage.

I second the   I always thought designing means doing stuff for a reason.

 

[emrr]

There is no particular reason why they are at this voltage.

That is funny; I take that to mean they personally know nothing about the design, since it was outsourced to a contractor (Hinson?).  The 'particular reason' would seem to be related to voltage regulator requirements, if I recall the rest of the thread correctly.  Coulda been different, isn't....

 

[emrr]

I suppose without real numbers it's impossible to be sure we know what each other is talking about.  It's interesting that a hot condenser won't overload it, that's not in keeping with typical american design. 

U-47's were padded pretty heavily for use with typical American preamps, it was a real problem.

It sounds like the problem is at intermediate level, between the two gain blocks, which would be the input to the output amp stage.  It also sounds like the sort of output headroom one finds in typical American preamps of that era, especially a modular type like this meant for console use, followed by a line amp.  Many American designs have input stage headroom that is equal to output stage headroom, and the output stages will overload grossly if input is near maximum. 

I personally don't like 12AX7's, and in this era I expect high amounts of NFB, resulting in unpleasant overload characteristics. 

I see this is indeed a modification/mash-up product, so some of my earlier commentary is off.  Maybe they did blow it in this one. 

Plenty of people pay tons of money for original units of the era, and it follows that there is a market for new manufacture that performs similarly.  UA's has catered to this market all along. 

I use almost nothing but 1940-1970 era preamps because I like the way they sound, and I expect them to work longer than any new product I can buy.  So I have a very different outlook.  They all require different working methods as compared to post-1970 designs, and they likely require a different approach than many popular european designs, especially those designed with an expectation of european condenser output levels. 

 

[Rossi]

Let's say the LA-610 is pretty much a near-exact repro of the original UA unit, for sake of argument.  From there I will argue it was never terribly 'popular' in the first place, as they really didn't sell all that many of them relative to the quantities moved by other manufacturers.  They had some historically significant installations (as did all the other manufacturers), and that appears to be the basis of the marketing pitch.  They appear to have sold far more of their modular 1008 and 1016(?) modules, from what I've seen on the used market.

It may be that I just don't like this design, but my feeling is, theres something wrong with the LA-610. As I said, I liked the (slightly younger) Solo 610 better, although is based on the same vintage mic pre, obviously. Also, there seem to be many board revisions (I think mine is revision M, and it's a fairly early one), and of course there is the MK II version. Some of the early models had problems with the P48 voltage.

 

[0dbfs]

I've got a rev J board in my mk I. I'll take some voltages later.

Thx,
j

 

[Pip]

I usually don't do this as it is not productive to be negative but:

Let me first state that I love UA gear I own a lot of the vintage stuff and I love the UA Audio plug-in cards (sorry if I broke a rule there).

I have both a UA Audio LA-610 MKII and a SOLO/610. They are not birds of a feather. The LA-610MKII Has been put away as it really has been one of the biggest session stoppers I have ever dealt with. It really sounds not good on anything but bass guitar and snare drum and then only OK. It is actually the only piece of gear I have ever had a singer say "I don't like the way that sounds" during initial playback. I was using, for informational purposes, my really good Neumann Gefell UM57 and at first I thought the tube or the capsule had died as the sound was so pinched. I think maybe, just to be fair, my LA-610 MKII might be broken but I have not chosen to deal with it because I  got a really good deal when I bought it (new) and I also plan on modding it at a later date anyway.

The SOLO/610 on the other hand is awesome and really sounds very close to a 610 with less noise. How can this be bad.

I will therefore stay tuned with interest!

 

[Rossi]

That mirrors my experiences. For the record: I own a lot of other UA gear that I'm happy with, sonically, including a UAD-2 quad with lots of plug-ins and two DCS remote preamps.

With the LA-610 there seems to be a discrepancy about the good stuff that's inside the box (lots of nice film caps, decent transformers, all tube circuitry) and what actually comes out.

 

[squib]

in the MK II, they use lots of relays, these are 5V relays and get their supply from the heater voltage.

is this the reason they chose 5V for the heaters?

if I take a tube and put it in my tube tester and measure its emission. I get a very different result on 5v compared to 6.3 V on the heater. How does this translate in a practical circuit?

 

[sr1200]

Pip, nothing wrong with giving love to the UAD platform.  Best sounding plugins on the market IMO.  As far as the 5v statement goes, I think thats kind of cheaping out.  We want to use 5V relays, so lets compromise the tube performance for sake of ease? Sorry that don't fly with me, and kinda makes me want to NOT use it based on principle.  With the crap ive had going on with my r0de mic and that pre and just looking into what this pre CAN do, I've really delved into the workings and settings on the pre and have come to the conclusion that the MkII can't be used like you'd use a "normal" preamp, especially if you're using the compressor going in.  One thing i constantly forget about (because its kind of under the lip of my studio furnature) is the gain switch.  I see that big input knob and think ok thats my gain knob.  Nope.  infact in the manual it states...

(7)Level-Determines the amount of signalsent to the final preamp output stage. For the cleanest,most uncolored signal from the LA-610, set the Gain switch (see #1 on page 4) to a low setting (-10 or-5) while turning the Level knob until the appropriate output signal is attained.

 
I normally would do the opposite of what that says.  I'd be setting the Gain to +whatever i need and using the level knob as a trim from there.
in addition:

The LA-610 Mk II differs from the orginal LA-610 with the tube used at the preamp outputstage,and now uses the 12AT7 just like the 2-610, SOLO/610 and 6176. This is a by-productof the newly implemented “true bypass” for the LA-610MkII, making the preamp sectionidentical to its cousins when in bypass.

and finally:

(1)Gain-Adjusts the gain of the input stage in stepped 5 dB increments. Turning the Gain switch clockwise raises the gain. Because this also has the effect of reducing negative feedback (see page 24), the Gain switch also alters the amount of the input tube’s harmonic distortion, a major contribution to the “warm” sound characteristic of tube equipment. The higher the Gain setting, themore coloration the LA-610 will impart to the incoming signal.

So take that for what it's worth.  I need to re-train myself on this piece.