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Use 8-Ch Power Amp as a Headphone Amp (Transformers?)

Discussion in 'Pro Audio Equipment' started by Randyman..., Oct 15, 2006.

  1. Randyman...

    Randyman... Well-Known Member

    What's up?

    I just picked up a Lectrosonics PA-8 8-Ch Power Amp for peanuts on eBay. It is a 6w x8 @ 8Ohm, or 10w x8 @ 4Ohms power amp of very high quality. http://www.lectrosonics.com/lecnet/pa8.htm (Retail over $1300)

    I want to use this as an ultra high-headroom Headphone Amp (4 stereo channels, all with their own inputs). I have already used it on my Remote Audio HN-7506's (same drivers as the Sony MDR-7506 - 24 Ohms Impedance and 2 Watts power handling IIRC). They get plenty loud - but I know that higher Impedance cans will invaribly be very quiet since this Power Amp has a limited Voltage swing (and I know the 7506's could still have more headroom, too). If my math is correct, the 24Ohm Sony's are getting roughly 1.6Watts per channel - and I know I could still use more headroom. But a 600Ohm set of cans would get roughly 0.06 Watts!

    I'd assume it is feasible to use Transformers to match the Power Amps' ideal 4Ohm load to the Headphones' higher impedance? And could you point me to some descent quality Transformers that would not saturate in the 10-15 Watt range? I only spent $200 on this amp, and I'd prefer not to spend hundreds more on Transformers if this is possible.

    I'm guessing something like a 1:4 "Step Up" or higher would be needed in order to drive high impedance cans like 600Ohm Senheisers, while still allowing me to use lower impedance cans through the transformer (anything beyond 1:6 would overload the Amp with a 24 Ohm set of cans, correct?). Do they make "Stereo" Transformers, or would I need 2 transformers for each set of cans? I guess I could buy 2 different transformer steppings for different impedance cans.

    The amp is happy sharing a common ground with the cans if that is of any importance. I was initially going to Bridge the amp into 4-Channels to avoid needing Transformers (more Voltage), but then I have no way of using the Headphone's Common Ground (Bridging the amp obviously makes both + and - "Hot"; so the "-" can not be shared as a common return). I believe Step-Up Transformers would be a better idea (but not free :( ), and allow me to retain all 8-channels (4 pairs) while maintaining sufficient voltage for the higher Imp Cans.

    Any expert advice on this is welcomed. This amp was such a good deal, I couldn't let it pass me by. It is totally useable w/o transformers, but all of my cans are 50Ohms or less - and I want to be capable of driving high imp cans, as well as get some more headroom out of this amp.

    Sorry I rambled so much - but that should cover everything :)

    :cool:
     
  2. dementedchord

    dementedchord Well-Known Member

    your over analyzing me thinks... the problem is not in providing enough current to drive phones... having a higher impedence requires less current not more... it's getting late and been a long day but it seems to me you should be fine as is... have you a pair of the other cans handy?? go ahead and try it you aint gonna hurt nothing... oh and great find man ...
     
  3. Randyman...

    Randyman... Well-Known Member

    :D I'm really happy with it so far. It has Euro-Block style I/O terminations, so I had to handle that accordingly (Pigtails for now). I'll probably mount 4 (or 8 ) 1/4" TRS jacks on a 1RU Rack Panel for the Headphone connections. There is another one from the same seller on eBay Right NOW (NIB)! Same starting bid of $200. I was the only one who bid on mine :)

    I know the current is not the issue - it's the Voltage. High Impedance Cans will need a good deal more than the 10 Volts this thing is likely set up for (6W @ 8Ohms = approx 7V at 0.9 Amps). Even if this amp could deliver 15Volts (I doubt it can), that would only translate to about 375mW into a 600 Ohm pair of cans - and any crappy HP amp can provide that (most HP amps are usually high voltage designed for high-Z cans, but not capable of any substantial current to speak of & crappy damping to boot).

    With a "Step Up" transformer, I believe it is stepping up the Voltage presented to the High Impedance cans while increasing the amperage drawn out of the low-voltage amplifier. Essentially - I would like to acheive the full-output power of this amp (at 4 or even 8 Ohms) but into a 24-50 Ohm pair of cans (for extreme Headroom). That should easily drive a 600 Ohm pair of cans with good headroom, too (I'd assume). If not - then I might need to find another transformer suited just for High-Z Cans (I can worry about that later). I'd hope something like a 1:4 or a 1:6 Voltage Step Up would end up doing fine with 600 Ohm cans (Probably still in the ~2 Watt range, and that should still provide some nice headroom).

    I have seen studios use the 100watt x6 Rane power amp for Cans! That is insane - but likely provides ample voltage for any type of cans.

    I just don't know exactly what Transformers would be best for this particular application. Any links to a suitable & affordable transformer would be greatly appreciated :cool: I've never had to source and select Transformers before! I'm so lost. Maybe something that could be mounted on the rear of a 1RU Rack Panel would be convenient? I'll probably start off buying one pair of Transformers - and see how they work - but which ones? :cry:

    Rock on 8)
     
  4. Kev

    Kev Well-Known Member

    100w is very big
    but it may also be for distance and a large passive network of cables

    I use a 40 w amp and it is for the voltage swing
    ... as you have suggested

    The step-up trafo has been used in the past and it can suffer some damping and resonance issues.

    Try it you might like it.

    The above unit may have swing up to 15 volts +
    but I'd need a schematic is comment properly.
    Test it yourself, to know for sure.

    Use a limiter before the HP amp and you can get the average levels up without the clipping.

    It's all a trade off.
     
  5. Randyman...

    Randyman... Well-Known Member

    Hey Kev :) Thanks for the reply.

    Looking at the only data I can find - its Power Supply spec's at +/-10V, and +/-12V (doesn't specify which ones are the Output Rails).

    I'd deeply appreciate any links or what not to something you think would be suitable for this application. I'm willing to dish out $20-$30 for a pair of them for grins - but that is about my max. I don't really know what is ideal here, and the 24 Ohm 7506 Sony Drivers are happy enough to make this "almost perfect", but not quite.

    Hey now :shock: I have been flamed pretty hard for using the term "Trafo" here before! :cry: Has it become an accepted abbreviation of Transformer - or are you "mocking" me? :p

    Thanks again for your reply, Kev!

    :cool:
     
  6. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    A 70.7 volt transformer used backwards will give you approximately 600 ohms. Here is what I am talking about http://www.rane.com/note136.html



    You can usually find 70.7 volt transformers at your local electronics shop and they come in various sizes from 5 watts on up. For what you are doing the 5 watt would be more than ok.

    There is much information here as well http://www.headwize.com/

    Hope this helps.
     
  7. Randyman...

    Randyman... Well-Known Member

    Thanks for your reply! I poked around on that Rane link, and over at Headwize, but I couldn't find anything with a "power amp transformer" search. What is the voltage step-up of these 70V Distribution transformers? I can't seem to find any details on the ratios (just wattage ratings). Would I need to worry about using 24 Ohm Cans with such a Transformer?

    I was thinking that anything beyond a 1:6 transformer would translate a 24 Ohm load into a >4 Ohm Load to the Amp - or does it not work like that? I know the basics, but I don't know any specifics about Transformers and how loading affects the load seen by the amp. I have 4 or 5 pairs of Low-Z Cans, and only 1 pair of High-Z cans - but I'd like to make this as future proof as possible (compatible with any cans down the road).

    I'd really like something I could use with the 24 Ohm Sony's, and something that would also work with the higher-Z cans if at all possible. If the 70V type Transformers would work - that would be killer!!!

    Thanks for all of your help.

    8)
     
  8. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    The reason I sent you to Headwize was to look at some of the technical information on phones and the tech section of the website.

    You want to bridge a power amplifier with the phones. Meaning that you want to be about 6 to 10 times higher in impedance than what you are bridging. The physics/electronic theorem states that maximum power transfer takes place when impendence are matched but today most phones are bridging meaning that a 24 ohm phone wants to see about 4 ohms to function normally. A 600 ohm phone want to see about 60 ohms put will work fine with 4 ohms. The main thing to remember is that the higher the impedance of the phones the less level you will have in the phones. (that is why a pair of 2000 ohm phones sound weak compared to a 50 ohm set of phones. The more phones you put across the output of the amplifier the lower in impedance you will get and since it is matched to the amp more the louder it will be. it is confusing and when you start getting into ohms law and bridging it gets even more confusing. Suffice it to say that you should use the same type of phones on the output of each channel of the amplifier and no one should plug or unplug the phones with the power to the amplifier on as you can momentarily short the output and that is something you do not want to do to a power amp. Without getting too technical you can use build out resistors for your phones which will keep the impedance more balanced and prevent them from shorting out the output. See the Headwize area having to to with technical matters.

    Hope this helps!
     
  9. Randyman...

    Randyman... Well-Known Member

    Wouldn't the "Bridging" aspect be looking at the Amp section (the "Output" section), not the Speaker section? The speaker could care less what it is connected to. From what I gather, this should affect how the amp controls the speaker (damping), but the speaker does not care if the Amp's Output Impedance is 0.001 Ohms or 1.5 Ohms - it is the Amp that matters IIRC.

    I never thought speakers were "looking" for any kind of specific output impedance - but the output impedance of the Amplifier will obviously affect Damping of the speaker. The Speaker's Impedance determines how much current will be asked for - but the speaker should not "Need" to see any certain source impedance that I know of (aside from the Damping effects I mentioned). A Bridging Line Input is also like this from what I know (The Output Section is what needs to be loaded properly - the Input Section does not really care what is feeding it IIRC).

    This amp wants to see 4-8 Ohms to get maximum power (10 Watts at 4 Ohms). So I need a way to present a 24 Ohm pair of cans as a 4 Ohm load to the amp if I want maximum headroom/power. Am I way off base? If the amp sees anything HIGHER or LOWER than the 4-8 Ohm Range, it will not be putting out maximum power, right? Since the Amp is LOW VOLTAGE - we need to optimize that Load to get the maximum CURRENT out of the amp to ensure we get the most power. Even if a step-up transformer could provide 50V to the cans, if the amp is seeing a 50 Ohm Load, then it is still limited to 1-2 Watts no matter how you slice it, right? Transformers don't "Create Power" they just shove Volts and Amps around to the desired places, right? 10Watts in = 10 Watts out (minus losses) - but the Voltage and Amperage will be different.

    I thought this was simply a way to Step Up the Voltage presented to the cans, while placing increased current draw from the amp? The amp's +12V Rails will not drive a 600Ohm pair of cans, but the amp has TONS of unused current just sitting there. If I step up the Voltage with a Transformer, then the 600Ohm Cans would see lots more voltage, and in turn end up with more Power. The amp is still putting out +12V, but the transfomer allows those 600Ohm Cans to draw more current from the Amp's low voltage Rails - increasing its total power. The amp needs to see 4-8 Ohms in order for it to put out its 6-10 Watts.

    I am not heavy into theory - but that is how I understand it. Am I wrong?

    I guess I will try some of those 70V Transformers, and see what happens. My primary concern is that the 24 Ohm Sony's won't present too much of a load on the Amp. I get foggy when thinking about Transformers, Step up Ratios, Primary/Secondary Impedance, and how that affects what load the Power Amp will end up seeing with any given load impedance. I don't see how this could work unless the amp is seeing a 4-8 Ohm Load (nominally with 25-50Ohm Cans is my target for now)...

    Anyone that can help me understand what I am missing would be a really swell guy! I seem to be missing something big... Sorry for being so dense! :)

    :cool:
     
  10. Kev

    Kev Well-Known Member

    not mocking

    Tech Talk (now defunct) concluded that trafo was the world wide accepted short for for transformer ... not TX
    The term is used all the time at The LAB

    Go to Headwise and to Rane for some tech notes.

    Voltage available.
    Current available
    Wattage Available

    you need a load to draw the current and hence the power ... in watts
    for a low imp loads like speakers or low imp HPs ... yes you have the 10Watts
    but for higher imp the wattage will drop because the load does NOT draw the current.
    A trafo can change the impedance match and change the way the power is transfered.
    ... you have changed the way the amp has been loaded.

    as I said
    just try it
    experiment
    you may like it


    Last point
    small Earphones and small Headphones are more voltage driven than current driven.
    Long ago Tweeters and Horns were not rated in Watts but were rated as 1.23 volt for 100dBspl at 4 feet ... for example
    these days we are more used to the 100dBspl/1W/1m
     
  11. Randyman...

    Randyman... Well-Known Member

    Alright. Thanks for the Trafo info! Maybe I will have to dig up that Thread where I was flamed for using that term, and shut them up :)

    So - I want 10Watts to be produced into a 24 Ohm Load. The amp puts out 10 Watts at 4 Ohms (roughly 6.32V @ 1.58Amps - I'd bet the amp does not have much more current than that). What transformer would allow the amp to deliver its 10 Watts to these 24 Ohm Cans? a 1:6 ratio trafo?

    I guess my major question is what SPECS do I need to look for in a Trafo? What Ratio, What Primary/Secondary Impedance (what is ideal here, and is the Impedance also dictated by the Ratios?), power handling (I'd guess 15 watts would be fine?). I'm lost TBH.

    On the Voltage/Current issue with High-Z Cans - I thought that was pretty much why HP Amps were "High Voltage/Low Current" designs. Most of them go into current limiting with 50 Ohm Cans well before the Voltage Rails are reached (a natural form of limiting the output with Low-Z Cans - Behringer's HP Amps come to mind ;) ), but the 600 Ohm cans would likely allow the full voltage to be achieved w/o running into the current limit of the amp. That pretty much allows fairly consistent POWER across Low-Z and High Z cans, right? (or closer than the situation I am in). If a HP Amp had a "High-Amperage" output stage (say 1 Amp), then a 50V HP Amp might be capable of 50 or more Watts into Low-Z Cans, but still only put 4Watts into a 600Ohm set of cans, right?

    In my case - I need a way to get te Voltage up high enough so the 600Ohm Cans can get 1000mW or so. With the Amp's 12V Rails, I believe a 600 Ohm pair of cans would only get 240mW. Not much.

    So - what wold be some ideal Trafo specs to allow me to use 24 Ohm Cans at full power (10 Watts), yet still allow me to retain 1-2Watts for 600Ohm Cans?

    Thank you guys so much for putting uip with my dumb butt! I feel I am very close to the "Breakthrough" that I am missing here...

    :cool:
     
  12. Kev

    Kev Well-Known Member

    :shock:
    maths and calculations
    :roll:
    bugger that
    just grab something cheap and start experimenting

    I know you want to get the max 10 Watts out of this thing but it may not be the best approach.
    !0 Watts out of each of the 8 will tax it to the max and I'm sure your averages won't need to be so high.

    try one of the small speaker trafos in reverse

    look here

    productLarge_1839.jpg

    you haven't got the 8 ohms but 24 and the line levels shouldn't come up to 100V.
    It will be a case of suck it and see.
    Trafos don't map out exactly the way you think when used in reverse because the reflected impedances can be a little unpredictable.

    Start with the amp into the common and the 5W (2.5K) terminal. HP hanging on the trafo inputs (backwards remember)
    This should give the lowest step-up.
    Then move up to the other end ... to the highest step-up.

    If you are using aligator clips ... be careful as you move between select terminals.
    Lower the volume as you do. Trafos can do weird things in step-up mode with instantaneous application or removal of volts.
    That's how an ignition system gets those high volts.
     
  13. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    Randyman...

    I think your way over doing the maximum current draw idea. You are NOT going to need 10 watts into a pair of cans. More like 50 to 150 Milliwatts. Go read the Headwize site. Do as Kev suggested and get a 70.7 volt transformer and EXPERIMENT. This is NOT rocket science. All the calculations in the world are NOT going to help you in this case. There are too many variables and you can't solve a math problem when you don't know the scope of the variables. Transformers used backwards can have unpredictable results.

    Chill a bit and just have fun.

    <by the way all the impedance figures you are working with are not set in stone and are only guideline numbers. With transistor gear the amplifier could care less about what it is driving UNLESS what it is seeing in the output is lower than it comfortably can drive given its current limitations. I would imagine that real output impedance of the amplifier is less than .5 ohms and the 4 ohm figure they are quoting is so that you do not overload the output of the amplifier by putting too low a load on it. In the world of tubes you had to match impedances much closer and you actually had taps off the transformer for different impedance. Transistor gear changed all of that.>

    Have fun and let us know how your experiments are coming.

    FWIW
     
  14. RemyRAD

    RemyRAD Guest

    Lectrosonics makes some very robust equipment. I don't think you should worry about output transformers or matching impedances for headphones to the amplifier.

    Regardless of impedance, every speaker offers a different efficiency. That is to say, even speakers of identical specifications may be louder than one another based on its efficiency.

    So if I were you, I would use that box connected directly to your headphones without any other interceding transformers. You won't blow the box up. It's designed to drive headphones, so drive headphones. My problem is, where is the ignition key??

    Driving Miss headphones
    Ms. Remy Ann David
     
  15. Randyman...

    Randyman... Well-Known Member

    Thanks for all of your valuable input. I think I have a good handle on what I wanted to do, but it appears there is no perfect solution.

    I will try those trafos ( ;) ) you linked to and see what happens (they are cheap enough).

    The amp is pretty loud when directly driving the 24 Ohm Sonys - but I know it could still have some more headroom (the Amp reaches its voltage rails WAY before the Current Limit is reached), and I just wanted a way to easily achieve this. I'm not worried about damaging this unit with high impedance loads (I understand Loading an Output and "Bridging" type Circuits, but I'm lost on Transformers), or worried about its current capability (this thing should handle full power all day long - but it is all about Headroom for me - so none of the channels would ever actually be at full power). I was more worried about presenting the Amp with a load BELOW 4 Ohms with a Transformer and 24 Ohm Cans - I get so lost with how the Load Impedance affects the Source Impedance through a Transformer.

    I guess I am over-thiking this - but all I wanted was some more headroom out of this amp - and I know it can do 10 Watts x8 all day long (this is a high-quality amp). Maybe I should look at each set of cans seperately, and optimize a channel for 24Ohm Cans, and a channel for 600Ohm Cans. Exactly like a Tube Amp has different "Taps" for different impedance loads - have one "Tap" for 24 Ohms, and a different "Tap" for 600Ohms. Maybe... Anyway - I'm sure I'll figure something out. Ideally - I'd love to raise the Voltage of the Output Rails on the amp - but I'm sure that is a huge project to even consider...

    Thanks for your time! You guys and gals rock :D

    8)
     

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