Topic: Step Up line transformer > line in or attenuator > mic in?

[ethan]

Ok I'm trying to put together my light weight travel rig and I've read what I can find on line transformers here. What I didn't see was any discussion about whether attenuating and going mic in is better or worse than amplifying and going line in.

I'm guessing step-up but want to see what others think.

Comments?

Basically I want to cover my ass in case I can't get full scale without them.

I'm planning on running:

ck91's or ck93's > MK 90/3+H 98 > SP-2 > SBM-1 > D8

The SBM-1's mic in is WAY too sensitive and the SBM-1 line in might not be sensitive enough.

So what would be best? A step up line transformer like the Hosa MIT-176 for line in or an adjustable attenuator like the the Audio Technica 8202 for mic in?

-e

[Brian Skalinder]

You're better off adding gain and running line-in than attenuating and running mic-in.

[JAH]

yup what Brian said....

[pfife]

I agree... what transformer are you thinking about getting?  I am having a similar situation going 4061 -> SP-SPSB-1 -> Mod SBM-1.  Ideally, I'd like to find a transformer that's 1/8" male on one end, 1/8" female on the other end... that maybe gives +10dbs or so...

[ethan]

I'm still researching there are a couple listed in the "Line Transformers" post.

I can see everyones point wrt to attenuation vs. gain and that was my first guess.

However, just to play devils avocate. Wouldn't whatever delivers the cleanest signal, introduce the least phase and lowest THD at the appropriate signal level be the best option even if its and attenuator? Or is the mic pre so worthless on the SBM-1 that this shouldn't even be considered?

anyhow I'm going to keep looking into it.

got to go watch 24 right now....bbl

[ethan]

Ok dug out the SBM-1 specs ad Line In is definitely the way to go given the THD, S/NR of all the inputs. But I still think all things being equal whatever gives the cleanest should be considered as an option.

-e

[pfife]

I'm actually talking with JKLabs right now about a fixed gain increase box.  I'll let you know what is decided!

[ethan]

Ok I'm getting closer to understanding this.

So now my question is the various line transformers all have variations of input / ouput impedance. How important is it to match the < 200 Ohm output impedance of my mics to the input of the transformer? Similarly my SBM-1's line in impedance is 47 KOhms. Also what's the difference between load impedance and electrical impedance?


[Edit] Found this: http://www.tape.com/Bartlett_Articles/impedance.html

So basically "bridging" source to load seems to be key according to this. So whatever transformer I get  I need to make sure it's input impedance is higher than my mics output impedance and it's output impedance is lower than the 47 KOhms of the SBM-1's line in impedance.


Hmm...this one looks sweet...no spec sheet I can find though.

http://www.tecnec.com/Product.asp?baseItem=OTL%2D1S&cat=INTERFACE&subcat=AUDINT&prodClass=IMP&mfg=&search=0&off=

Here's another interesting one:

http://www.sweetwater.com/store/detail/CleanBox/

-e

[jk labs]

I'm still researching there are a couple listed in the "Line Transformers" post.

I can see everyones point wrt to attenuation vs. gain and that was my first guess.

However, just to play devils avocate. Wouldn't whatever delivers the cleanest signal, introduce the least phase and lowest THD at the appropriate signal level be the best option even if its and attenuator?

The latter statement is true.  An attenuator in itself is a near perfect device. A mic pre can be made to have excellent specs (but it's noise performance might suffer grossly with an attenuator in front). A good transformer might be an asset to you sound.

So it boils down to comparing the actual parts you are facing. Most people seem to think the mic-pre in the sbm-1 is best avoided :-) Hopefully you can find a transformer with better specs.   

You are running with 300 bodies no? Sticking a smallish +10 .. 20 dB transformer in the chain will not upset impedances here.

[ethan]

So I just sent Tecnec email asking for specs on the "On-the-level Low to High Z" unit....sure looks like it would be great given the dual input and RCA outputs.

What is it with all these manufacturers not publishing specs....wtf...couldn't find anything for the Shure or HOSA units either.

-e

[ethan]

Here's the specs on the Tecnec Low to High Z OTL-2S:

"The impedance specs are indeed 600 ohms on the balance side and 10K on
the unbalanced side. We have not run any test for the SNR, THD or dBu
specs.  The Frequency response is 1K -10K @ -1dBm."

So now I guess my question is can I compute gain knowing the Step up values of impedance? Is 10KOhm output impedance to low for my SBM-1's 47KOhm input impedance?

-e

[leegeddy]

Here's the specs on the Tecnec Low to High Z OTL-2S:

"The impedance specs are indeed 600 ohms on the balance side and 10K on
the unbalanced side. We have not run any test for the SNR, THD or dBu
specs.  The Frequency response is 1K -10K @ -1dBm."

So now I guess my question is can I compute gain knowing the Step up values of impedance? Is 10KOhm output impedance to low for my SBM-1's 47KOhm input impedance?

-e

i'd go with a Hosa MIT-176 (50-50k)

marc

[ethan]

Here's the specs on the Tecnec Low to High Z OTL-2S:

"The impedance specs are indeed 600 ohms on the balance side and 10K on
the unbalanced side. We have not run any test for the SNR, THD or dBu
specs.  The Frequency response is 1K -10K @ -1dBm."

So now I guess my question is can I compute gain knowing the Step up values of impedance? Is 10KOhm output impedance to low for my SBM-1's 47KOhm input impedance?

-e

i'd go with a Hosa MIT-176 (50-50k)

marc

Well the problem I see with the HOSA is it's output impedance is 50K and the SBM-1 (load) will be 47K this doesn't provide the approapriate bridge and afaict would affect frequency response.

-e

[jk labs]

Transformers are coupled inductors. They are specified by such parameters as inductance, winding ratios, frequency response(!), max current before core saturation etc etc. What transformers do is to CONVERT the impedance from one port to the other. In so doing they change the voltage as well. The Bartlett article contains the formulaes no?  If not think conservation of energy (UU/R) between input and output.

You're interpreting the HOSA spec too critically. What Hosa tries to say is that if the load impedance is much lower than 50 kOhms
the core is approaching saturation (seen as brickwalling on the output side). The sbm-1 line in is probably well within what the Hosa can drive to specified levels.

[Chris K]

fwiw, i ran 

391 > b18 > line transformers (audio technica cp8201) > sbm-1 line in

when i first started concert taping, and made some great tapes.

here is a link to the audio technica site with the line transformer specs. you can get them at guitar center for about $20 a piece.

http://www.audiotechnica.com/guide/other/access/inline.html#CP8201

[ethan]

I found the formula here:

http://www.shure.com/support/technotes/app-transform.html


Gain = 20 * log (sqrt(ImpedanceOut/ImpedanceIn))

So the Tecnec box is a approximately a 12 dB gain.

-e

[redbook]

do these AT transformers provide gain? Same levels as Hosa ones?

[ethan]

Basically they match impedance but the affect of matching 600 Ohm -> 50 KOhm is they provide gain up to 25 dB from what I've read.



BTW, I've decided to give the "On-the-level" line transformer from Tecnec a try. I'll let ya'll know.

-e

[hexyjones]

 An attenuator in itself is a near perfect device. A mic pre can be made to have excellent specs (but it's noise performance might suffer grossly with an attenuator in front). A good transformer might be an asset to you sound.

That statement seems to be at odds with itself...? (perfect device/suffers grossly)?

Also - is this really "adding gain", or just changing the way the signal looks to the subsequent device...?

Would there be any loss/distortion along the way in the transformer...?(even though the recorder might see more gain)

(I probably dont fully understand the concept of gain...electrically speaking...beyond the obvious..)

[ethan]

Read that impedance FAQ...it'll answer some questions for ya.

-e

[Todd R]

I also used the AT transformers mentioned above when I first started taping.  Went line in to an Aiwa HDX3000 DAT (same as the marantz brick and Aiwa HHB1) and made many great tapes.  I lost those transformers and used the Radioshack version for a few shows, but then moved to a beyer MV100.  The ratshack ones were ok, but I'd recommend the AT ones for sure.

[Todd R]

BTW, I've decided to give the "On-the-level" line transformer from Tecnec a try. I'll let ya'll know.

Didn't you say the freq response on these was 1K-10K?  I'd go with the AT or Radioshack ones (20-20k and 50-20k) before going with ones with such limited range as 1k-10k.

BTW, I've got the RS ones packed away somewhere.  You can borrow them if you'd like.

[ethan]

Todd the Tecnec one has a 20 Hz - 20 KHz range.

The 10K refered to the output impedance.

-e

[Todd R]

The Frequency response is 1K -10K @ -1dBm

Just checking -- so this info you posted before was for a different TecNec transformer?

Also, gotta nix that offer for borrowing my RS transformers.  I just looked for them, but can't find them.  Must have lent or sold them to someone else, though it escapes my mind to whom.

[jk labs]

 An attenuator in itself is a near perfect device. A mic pre can be made to have excellent specs (but it's noise performance might suffer grossly with an attenuator in front). A good transformer might be an asset to you sound.

That statement seems to be at odds with itself...? (perfect device/suffers grossly)?

Also - is this really "adding gain", or just changing the way the signal looks to the subsequent device...?

Would there be any loss/distortion along the way in the transformer...?(even though the recorder might see more gain)

(I probably dont fully understand the concept of gain...electrically speaking...beyond the obvious..)

A transformer is a passive device: You cannot get more energy out of it than you put in. But the transformer can amplify the voltage (step-up transformer).  Now total energy (voltage x current) is conserved so you get correspondingly less current out.

Most all audio circuits these days measure the voltage. So when a device has a gain of say 20 dB it's implied that we are talking about the voltage. A transformer with 20 dB gain (meaning voltage gain) is a 20 dB attenuator of the current. 

Audio transformers are not ideal devices. It cannot pass DC. At high frequencies the core material sets limitations. I think you’ll find near infinite amounts of info on the net on this very topic. 

A way to visualise voltage and current is to think of voltage as water pressure and current as the amount of water flowing per second. 

[hexyjones]

Thanks - that makes lots of sense now...thanks.

Does that apply for both balanced and unbalanced lines alike...?

Also - isnt the quality of the transformer pretty important?...at one point someone was reccommending using Radio Shack transformers - which I was always told - sucked...and were to be avoided. Dont these things need to be matched to some extent - like mics?

I thought maybe that advice needed to be reconsidered...?

[jk labs]

Thanks - that makes lots of sense now...thanks.

Does that apply for both balanced and unbalanced lines alike...?

Also - isnt the quality of the transformer pretty important?...at one point someone was reccommending using Radio Shack transformers - which I was always told - sucked...and were to be avoided. Dont these things need to be matched to some extent - like mics?

I thought maybe that advice needed to be reconsidered...?

Hi there. Balanced line you say. The transformer doesn't care (nor does it have any way of finding out) if one side is at ground or is fed a signal of inverted phase. So the analysis does not change. 

on the issue of recommending RS transfomers, or calling them crap.
I think this is exactly like all other gear discussions. It comes down to what you need/want/can hear/enjoy/can afford ..   People don't put a lot of cosmetics into transformers so there is a correlation between cost and measured performance.

The only thing that really matters is what you are happy with. A working transformer needs not cost much. You can even make your own easily. But it will affect the lows and the highs and add it's own sonic footprint. I'm sure that some will say "wow, sounds good in my system". Maybe attenuated highs and rolled off bass is  just what is needed. Then the mid-band colorations seem a small price to pay.

Regards
Jon

[hexyjones]

well - I admit - I own some Shack transformers and might use them occasionally...

A mentor of mine is the source of the "crap" comment - he thought it was hard to get a pair that sounded alike...

So - keep an eye out for stuff like this?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=3278&item=7303101938&rd=1

Do you have any preference? the sonic footprint of a transformer or the noise penalty with the attenuator...? or is this, as you said, "what you need/want/can hear/enjoy/can afford"?

Thanks for all the helpful info!

[jk labs]

well - I admit - I own some Shack transformers and might use them occasionally...

A mentor of mine is the source of the "crap" comment - he thought it was hard to get a pair that sounded alike...

So - keep an eye out for stuff like this?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=3278&item=7303101938&rd=1

Do you have any preference? the sonic footprint of a transformer or the noise penalty with the attenuator...? or is this, as you said, "what you need/want/can hear/enjoy/can afford"?

Thanks for all the helpful info!

You're asking the wrong person. I rip things open to see how I can make it a perfect match for the device sitting downstream. Then I go to the device downstream...

In general you want to get the signal amplitude *up* as early in the signal chain as possible. Then downstream you provide just sufficient gain to reach 0 dB FS. Going through stages with 0 dB gain is not optimal as I see it unless you have a good reason for so doing.   

I.e. I do agree with those who recommended an external transformer ahead of the line stage instead of going attenuator + mic-in & then line stage of the sbm-1.  I hold this view irrespective of mods done to the sbm-1.

I encourage experimentation. Buy those Sowters and try (if they provide ballpark 20 dB gain and handles 50 kOhms loads [which I think they do judging from their size]).

Jon
PS Of course the RS xformers are crap, especially in the bass. We must assume they are produced for RS by the lowest bidder in the entire world. Every penny they can save on materials, production and quality control contributes to their profit. But remember that the sound tapers capture have gone through a complex signal chain including speakers with considerable TDH and deviation from flat. And maybe the mics used were omnis with excessive bass. So it must be YOU who decide if the RS transformers are ok to use in YOUR setup. Sample to sample variations may make their use somewhat more of a challenge :-)

[Todd R]

Just to clarify, since I think I was the only one mentioning RS transformers in this thread.  I am not really recommending them.  I'd recommend the AT transformers that I used for awhile.  But I only picked up the RS ones in a pinch after losing the AT ones.  Ran the RS ones only a couple times before I switched to a Beyer MV100 preamp.

I would however, as I mentioned before, use the RS transformers before I used a pair of transformers that only provided a signal in the 1000Hz-10,000Hz range (assuming that information is correct).

Jon -- thanks for all your input on this.  I'm not following you in regards to your statements on transformers and the SBM1.  If you are better off stepping up the signal amplitude earlier in the chain, wouldn't that argue for using transformers rather than attenuators?  Also, the unmodified SBM1 uses a 4580 op-amp based gain stage for the mic pre input and another 4580 gain stage for the line input.  So a transformer-boosted signal going line into the SBM1 would go through the transformer gain stage and then the single 4580 gain stage in the SBM1.  If you went the attenuator> mic-in route, you'd be going mics> attenuator> 4580 gain stage> 4580 gain stage.  So by using the transformers, you skip one 4580-based gain stage, opting instead for the line transformers.  I have no idea whether line transformers would sound better than a 4580 op amp, but it seems that if they do there might be merit to going with the line transformers rather than going through an attenuator and then mic in.  I'm curious why you'd always prefer the attenuator > mic-in route.

[ethan]

I would however, as I mentioned before, use the RS transformers before I used a pair of transformers that only provided a signal in the 1000Hz-10,000Hz range (assuming that information is correct).

Todd I agree...however they were claiming that the singal was -1 dB in that range so I assume that that's the optimal range and that it just might be slightly less. I doubt the Tecnec product is that limited. It apparently is used quite a bit  in audio production from the film industry.

-e

[Todd R]

Oops!    Yep, totally misread that -- I saw it as you don't agree, not you do agree.  Makes perfect sense now.