Langevin AM GC 17

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
ok- attenuater mystery solved - i started researching different attenuator configurations to figure this out (and to learn how they work in the process) and realized the mistake in my second drawing :oops: if you reverse the conections of the 2 680ohm resistors (mirror them top to bottom) you have an un-balanced bridged-T attenuator, like so:

btpad.jpg


which, when looking at the actual wiring, is indeed what is there. R3 and R4 are the two halves of the ganged 40k pot, R1 + R2 are the 680ohm resistors.

following through, i wired the input up unbalanced and presto: crazy attenuator hum gone!

thanks for proding me toward understanding this, everyone.

now, a question: i have the output wired balanced, which works great. in order to do the same for the input, would it simply mean replacing this attenuator with the same values in a balanced bridged-t configuration?
 
I put mine up on the bench tonight and disconnected the t attenuators and now they sound pretty happening. They sound pretty hissy though, I imagine this is just a byproduct of the design of the thing? They also draw ridiculous current, with a half amp supply, two wont work in series... Seems like alot of power, but I guess thats what you get for using your power amp as a mic pre...


edit: on mine, the hi-power strap is just the hiss maker. I dont get any additional gain but a TON of hiss. I dont know if this is indicative of the design or the state of age of my units, but both behave equally. At the low power setting these sound a lot like am-16s, but have a slightly sharper high end. should be cool to check these on the next record for sure.

dave
 
> you have an un-balanced bridged-T attenuator

Yeah, but "balanced" is relative. It is going into a floating input (transformer winding), right? If the wires are short, the fact that the attenuator is asymmetrical does not matter. (It isn't truly unbalanced: it has no ground connection.)

It should accept a balanced or floating input no problem. If you are coming from a 3-pin microphone, connect the two "hot" wires to T and U, connect the third (ground) pin to the chassis.

> the hi-power strap is just the hiss maker. I dont get any additional gain but a TON of hiss.

The high-power strap should NOT affect the gain, only the maximum output.

I find it hard to believe you would ever need Hi-Power in a small studio (unless you have 150Ω loudspeakers), but it should not increase the hiss either. Something is wrong.
 
PKK- regarding the unbalanced vs balanced input, i was reading about this in your post on the "am-16 on my bench" thread. i think i understand why this shouldn't matter, but....in my first attempt at connecting this, i wired it as you describe; hot wires to U and T, and third pin to ground. in this case i got quite a bit of hum with the attenuator; increasing linearly with the amount of attenuation applied. when i wired with the (-) side of the output of the mic to the ground (to "un-balance"), this hum went away. is there something else wrong with my grounding scheme perhaps, as....

my unit has (what i imagine is) the same hiss that you have, Soundguy. however, it doesn't seem to get worse with the high power strapping, just louder.

i was also wondering about the current draw - it seems huge compared to other amps with a similar amount of gain - are these really even worthwhile to use as a pre-amp, or better left to driving speakers?
 
bump...

Electrog, Have you found your AM17 to be usable?
I've got two waiting to be hooked up.

Also, have you found connectors for these?
 
I disconnected the attenuators from mine and they are fine, they just draw a TON of current. They'll work fine as mic pre's just make sure you have a big enough power supply, I get the feeling they want at least 1 amp a piece... I have a 1.6 amp supply and it wont power two...

dave
 
[quote author="soundguy"]I disconnected the attenuators from mine and they are fine, they just draw a TON of current. [/quote]

Cool, are you using your own switched attenuator on the input and/or on the output?
 
I suspect that I'll put a balanced attenuator on the input only, I havent decided what to do with that box yet, Im racking like 4 things right now waiting for parts, so we'll see how it works out. I am hoping to have it done in two or three weeks if you'd like to check it out. I just took the input transformer leads and rewired them to the connector and then removed the front panel and attenuator all together. They'll fit snugly in a 2 space par metal case, Im a little worried about heat, so I'll probably drill holes in the top panel. have them in a box with a pair of altec 9470. I need to figure out a monster supply for these though...

dave
 
jvanslem, i have hooked the one i have up to test.

in short: it sounds good enough in the limited use i gave it to make me want to rack it up properly, however, there are a few hiccups that still have me wondering.

as soundguy stated, these things draw a ton of current - i only have one, so i think i'll be ok with the psu i had intented for it (which will supply roughly 1.5A) if i run at low power (which seems to be plenty from my testing). two units would require something beefier, at least 2, if not 3A, if the stated specs are correct.

the bigger issue (which hopefully someone can point me in the right direction on) is that my am17 is quite hissy. like soundguy, i solved some of my bigger buzz/hum issues by removing the pad (and to a lesser degree by changing the wiring on its input, as mentioned above), but a destinct hiss remains. which, if i can't solve, will make for a good souning but unusably noisy preamp. what could be causing this? tired components?

david
 
[quote author="soundguy"]I suspect that I'll put a balanced attenuator on the input only, I havent decided what to do with that box yet, Im racking like 4 things right now waiting for parts, so we'll see how it works out. I am hoping to have it done in two or three weeks if you'd like to check it out. [/quote]

I may take you up on that... at least it'd be cool to see digital pics of the end result.


[quote author="soundguy"]I need to figure out a monster supply for these though...[/quote]

Yeah, sounds like a lot of power going on there. I've got a couple weeks before I get to mine so I may just look for a Power One on ebay.
 
...on another note, thinking that i may have to do some more intense investigation (or repair) to get this working properly has driven me to sort out the differences in the unit i have with the posted scematics. turns out the one i've got is essentially a hybrid of the two schematics at the begining of this thread. i'd be happy to run through it if anyone is interested.

if i may ask; soundguy, have you checked yours against either of these schematics?

i'm becoming quite curious about the history of these things. the 16 and 17 are clearly based on the same basic design. i've never seen a 17 with the same form factor as the 16, though. were these ever side by side products, or did the 17 come later, as the styling seems to suggest? anyone have any history they'd care to share?

david
 
i havent compared it to the schematic at all. it definitley matches the frequency reponse curve posted though...

dave
 
I undid the high power strap and all the hiss went away. The noise that is left now is just poor grounding. The am16, the 9470 and this amp are so far all very particular about their grounding, but after much struggle, I got my 16's pretty quiet, Im assuming I'll be able to do the same with these. I have no idea what that hiss was about and with all the other shit on my bench, Im not too inclined to find out so long as it works now. If its something breaking, I'll deal with it when it fails... Shitty attitude I know, but I just have so much else Im trying to power through right now.

dave
 
The posted schematic has two transistors paralleled in the output stage that do not appear in the Audio Cyclopedia schematic. The units I've seen have been like the Audio Cyclopedia version, and had a pair of power type output transistors.

These were offered as the program / monitor amps to go with the AM-16's. The earliest versions were not as wide, and I'm guessing they had heat problems, so they became larger somewhere along the way.

Re: the 'max output power' strap. The gain stays basically the same, and the input and output headroom moves. The noise floor may change as a result, but if you need more input headroom, then the higher input level of your source may mask the added noise. Why it's so dramatic a change in these, I can't say.

This headroom shift is true for the AM-16 and the Altec 9470-A. Probably also the tube AM-5116-B and AM-5117.
 

Latest posts

Back
Top