D-LA2A Support Thread

Thanks sirko ,
                      I figured it was something like that  ;D

I've been wondering how Drip's new T4 compares with the old.  The old one seems to have a steeper compression curve which isn't really that desirable, from what i understand.

desol said:

I've been wondering how Drip's new T4 compares with the old.  The old one seems to have a steeper compression curve which isn't really that desirable, from what i understand.

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I've just tested the fast one yesterday and we're all pleased as punch here! tested against a NOS UREI 1970 for reference. It sure is faster, but just as optoish and "nice".

great info , Thanks 

I finally settled on a piece of analyzing software and decided to purchase TrueRTA, in order to learn to measure my gear. I hope you guys don't mind me posting a few shots so i can maybe get some help with basic interpretation. Here are 4 quick sweeps with my Dla2a, with the gain reduction progressively increasing from zero to around 65% reduction and one square wave input at 1k. I noticed that it really doesn't start to do anything until the peak reduction knob is at around 35%. Seems to have a bit of HF peak up around 18k. I used edcors on the output, no zobel installed. The unit looks to measure fairly well in terms of freq response i think, and i don't mind the peak at ~ 18k...adds a slight shimmery quality despite the compression(hf reduction). The square wave looks a little off and i'm not quite sure how to interpret the overshoot/ringing on both the leading and falling edge?. Anybody have any advice they can offer me? Thanks.

Good questions - not sure myself  ???

The LA2A should begin limiting fairly quickly and remain in limiting part of envelope for the whole duration of True RTA sweep

(around 1s or so from low to high with level as set by True RTA's signal generator section)

So the display at the high frequency part is being influenced by the limiting that started earlier at the low frequency part of sweep.

Be interesting to make frequency sweeps band limited and then aggregate the results so as to see more clearly the spectral response of the opto.

I mostly try to 'see' the response of a limiter using a DAW where I prepare a test signal track comprising of pulses of varying amplitudes and varying durations and different constant frequencies.  Then try to infer something about the frequency response under limiting

Or not 

It's a complex subject and I've never really gotten much satisfaction from frequency sweeps with limiting action enabled. Limiter's response depends very much on the shape of the signal.

Be great to have some light shed on this subject.

alexc said:

Good questions - not sure myself  ???

The LA2A should begin limiting fairly quickly and remain in limiting part of envelope for the whole duration of True RTA sweep

It's a complex subject and I've never really gotten much satisfaction from frequency sweeps with limiting action enabled. Limiter's response depends very much on the shape of the signal.

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Hey Alex. Thanks for your response...as you can see i took your advice and picked up trueRTA. Level 3. Glad you recommended it. What i meant to say was, i increased the amount of reduction from 0 to about 65% by turning the peak reduction knob for each graph. So yes...the compressor did engage properly and hold during each sweep. I'm kind of concerned about the square wave tho. Freq response looks ok...i think...but square wav looks sorta mangled. 

Maybe it's not so bad? Maybe it's distortion and hi freq lift? I need to learn what the square wav's are telling me.

Oh btw, square wav is without any reduction...just straight through w/amplifier engaged.

Ok. Let me take a stab at this.



The slanted rising and falling edges indicate that the amplifier is a little slow in terms of slew rate; the speed at which it can reproduce the square wave. I'm not sure if anything can be done about this, as it may be a part of the inherant design. The overshoot/ringing on the corners is the over-emphasis of the high frequency response, which is also noticeable in the freq response graph above, at ~18khz. This means that the output of the amplifier needs to be dampened slightly by adding a zobel network(what this can also tell in terms of the input transformer, i'm not sure). The inclination of the ceiling and floor of the square wave towards the centre, indicates that the amplifier isn't reproducing low freq? as good as it could(i read this)...but it isn't too bad. Again, i'm not sure if this is the fault of the output/input transformer or both. Anybody care to add, correct? I find this super interesting..

The square waves don't look too bad to me.
Yes there is some ringing and some not perfectly horizontal tops and bottoms.

Many audio units show this sort of thing and can be minimised with some attention to the zobels and such.

Basically I agree with your explaination except I don't think that correlates to what the frequency sweeps show. I could be wrong tho'.

I would also compare these with a CRO to ensure what you are seeing is not an artifact of the analyser.
Square waves are a bit of an approximation with sound card interfaces, particularly at non HD sample rates.

PS - Glad you're liking the analyser. Set it to max speed and max resolution and use it to optimise noise performance by positioning of traffos and wires.

Then try to find some theory to explain the observations!

Real 'butterfly effect' stuff.

Cheers

Hi,
I tested my Rude Tube with a square wave and had almost the same results as shown in your picture. Lots of reading about zobel network and transformer ringing didn´t give me a solution. Finally I took the unit out of the loop and connected input with output xlrs, just to check that it really is the tube amp that causes this strange waveform. In fact, it was my soundcard that was not able to reproduce an accurate square wave. You might want to check this.
regards
Bernd

Thanks Alex and Bernbrue for pointing this out. I'm using an mbox3 to and from the DLA2A.

- Indeed, it is the mbox, apart from one small difference. It would be nice to build a square wave/sine wave signal gen, with switchable 150/600ohm output impedances, and xlr connectors.

I have a maybe stupid question:

I've nearly finished my PCB but did solder the 7805 standing up. How much cooling does it really need? Will a normal heat sink be sufficient?

So far I've only seen pictures where it's connected with the case. I mean it's only used in the relais, isn't it?

sonic,
It does get surprisingly hot, originally i had a little clip on heatsink but changed it for a larger one to be safe.

Thanks! Do you have a pic of your current heatsink?

I planned to use one of those, but it will be tough to connect the nearby connectors then:


Otherwise I would have to desolder the 7805 and solder another 7805 instead because the legs are too short now.

I think thats exactly what i used.
Mine's about an inch wide.

I layed the regulator horizontal and supported the heatsink weight with a piece of plastic coming off the pcb mount next to it.

Crappy photo, but here's mine.  The heat-sink I pulled from an old TV PSU, and the IC runs very cool.  It's a lot beefier than the typical clip-on bolt-on that I see on a lot of regulators.

PS, old busted/failed household or other electronics are an awesome source for parts like these if you don't mind keeping a bone-yard around.  It's sure saved me shipping from mouser etc several times.

I bent the IC horizontal for the wiring, and then bent it back vertical once it was complete, just don't tweak it so much you bust a leg.

Thank you both, that was very helpful! 

hey
im finally almost done with this. i have three questions remaining:

- for d101, d102 etc it says to get a 1N4141 but using the part number i get an 1N4148. is this the same part or interchangable?

- do these go right next to the relais? i dont have their positions marked on my board..

- can anyone explain to a dummy like me how to wire the board properly with utc a10s and a24s?

thanks a lot, i appreciate any help.

jakob

salomonander said:

for d101, d102 etc it says to get a 1N4141 but using the part number i get an 1N4148. is this the same part or interchangable?

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1N4148 is the correct part (where does it say 1N4141? if it is in any of my posts I will edit the typo ...)

salomonander said:

do these go right next to the relais? i dont have their positions marked on my board..

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yes

Can anyone help me? I'm not understanding the way this thing is suppose to operate?

1. I've never owned or heard a LA2A operate so I'm not sure of it's character or if this is the way its suppose to sound. But It seems to be a Distortion compressor more than anything. As I turn up the reduction knob, It really doesn't do much until it's well past half way up, and by then it's distortion. Is it that  that I'm not hitting it with signal hard enough? but when I do hit it hard to get some gain reduction and say a vocalist is singing but then gets loud, it distorts bad! Is this normal? I'm not sure what may be wrong or if there is anything wrong with the way it compresses. It's not much at all but when it does it's distortion.

2. I can't seem to calibrate the meters right? I'm using a 02R96 to run it's osc through the la2a and back to the 02R96. With the limiter knob all the way down and I try to match the bypassed signal with the gain, I can't without the meter being pegged well beyond +4 and then I can hear the osc. distorting?!? There seems to be a 4-6db difference when it's in bypass and when it's on? Like when it's in bypass, the signal is fine, but when I flick the switch from bypass, it's 4-6db lower in volume, but the meters read +4?? Also, when the meters are in limiter view, they go positive before going to reduction? And the needles move REALLY slow? but when the meters are reading output, the needles move fast.

I'm using Hairball 8020 meters with a 3.9k resistor running in line on the + terminal. I am also using a 100K pot for RX25. I hooked it up with pin 1 of the pot to the one through hole for RX25 and pins 2 and 3 through the other hole for RX25. Is this correct? Oh! and I'm using the recommended Sowter input TX and Edcore OT.

Please!! any help will be appreciated!!