'100V' Audio Amplifier - Any Experts? - Page 1
Forum section: electronics / general

author: ed_reardon
author_rank: Regular Contributor
timestamp: June 03, 2017, 08:14:13 am
content: Hello all,In a true case of not reading the eBay description properly I have landed myself a 'slightly faulty' (noisy pots) 100V audio amplifier and mixer (). Thankfully it cost less than a few pints of beer for both of them and I can gut them for plenty of useful odds and ends if they're useless.I am involved in a live music event for a late friend and I bought this hoping I could fix it up and make it useful for the day, I assumed 100V spec' meant something to do with phantom power and dutifully ignored it, turns out I'm very, very wrong. 100V refers to the P-P output signal, and it looks like it's designed for large background music systems with very long cable runs, then stepped down at the speaker.A few questions:- Would this be in any way compatible with conventional 'PA' type speakers like you may see in a small pub for live music?- The amplifier also has a 24V 'tap' for the speaker outputs, is this usable with any conventional 'PA' speakers?Or indeed anything else useful you may wish to throw my way!Essentially can I get any use out of this thing before I gut it for parts? I know the questions are rather nebulous!Cheers,Ed

author: Brumby
author_rank: Unknown
timestamp: June 03, 2017, 08:38:45 am
content: It's been a few years since I played with PA gear - but 100V is, indeed, from the power output of the amplifier - for the exact reason you mention - long cable runs. There are also 70V and 25V lines. It is not uncommon for line transformers to have 70V and 100V primary connections. I haven't seen much in the way of 25V setups.The amplifiers I've worked with have traditional low impedance output stages that are then fed into a step-up transformer. The resulting 100V line is then tapped at each point for a speaker with a step-down line transformer. Selection of this transformer will be dependent on the line voltage, speaker impedance and power required.The answer to both questions is: Compatibility with any speaker will depend on whether it has a line transformer that matches the line voltage. It is common for the transformer to be inside the speaker enclosure.

author: Brumby
author_rank: Unknown
timestamp: June 03, 2017, 08:51:35 am
content: Some helpful stuff here:https://en.wikipedia.org/wiki/Constant-voltage_speaker_system

author: The Soulman
author_rank: Super Contributor
timestamp: June 03, 2017, 09:28:45 am
content: I don't like the "constant-voltage" expression it is very confusing for the novice,I've seen people trouble shooting a 100 Volt system and they quickly concluded that the amplifierwas broken, at background level music it didn't output 100 Volts..These days 100 Volt systems use transformers at both ends to transform low impedance (4-8 ohm)/low voltage/high current signal to high impedance/high voltage/low current signal.The lower current results in lower voltage drop across the speaker cable and the higher voltage tolerates higher voltage drops resulting in a much more efficient energy transfer.Also typically the transformers at the speakers have different transformer taps so it is possible for the same type speakerson the same cable to play at different sound levels, very useful when you have a large high ceiling room (store) and a small low ceiling room (toilets).To answer your question, most 100 Volt amplifiers have a separate output connection to drive (low impedance 4-8 Ohm) regular PA style speakers, have a look at the connections.Or post some pics or model numbers of these amplifiers.

author: Brumby
author_rank: Unknown
timestamp: June 03, 2017, 09:36:55 am
content: Quote from: The Soulman on June 03, 2017, 09:28:45 amFair comment. I know I was scratching my head when I first encountered them, but logic soon sorted out how ithadto work.QuoteI suppose you could find such people ... probably more easily than you might like.

author: ed_reardon
author_rank: Regular Contributor
timestamp: June 03, 2017, 10:00:01 am
content: Cheers all.Yes it does (having taken the little cover off the terminals at the back) have an 8-ohm output.However seeing as I paid £4 for it I keep looking at the innerds thinking it would make the basis of a fantastically powerful bench power-supply!The amp stage is fine so that'd make a useful assembly to have knocking about in the junk box (nice heatsink and big BJTs), and transformers are always fun to experiment with.What I have found which has interested me, I assumed that the low impedance (nominal 8 ohm) output would bypass the output transformer but it's sitting across the transformer, I am guessing something to do with impedance matching (perhaps pulled off 'very early' on the secondary?)Cheers for your help guys.The tinkerers curse, I've found something easy to fix, but now I want to turn it in to something else

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2017, 11:53:47 am
content: The 8 Ohm output will most likely go to the primary side of the transformer. When the 8 Ohm output is being used, the transformer will still have a voltage applied to its primary but it won't do anything, other than draw a small magnetising current.I don't see how the amplifier would be much good as a bench supply, unless you want it for outputting various AC voltages, since both the input and output stages will be AC coupled and will block any DC signal applied to the input.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 03, 2017, 03:02:24 pm
content: Quote from: Hero999 on June 03, 2017, 11:53:47 amRemove the output transformers and hack it to be DC coupled. Then you have a 4 quadrant PSU, just beware that continuous rating is much lower than audio rating.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2017, 03:27:54 pm
content: Quote from: NiHaoMike on June 03, 2017, 03:02:24 pmIt's quite likely that will work but there's no guarantee that it will. It's possible there are elements, such as emitter resistors bypassed with capacitors, which won't play nicely with DC. If it's a modern amplifier there's more chance of it being able to be hacked for DC operation, than if it's old.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 03, 2017, 06:21:45 pm
content: Seems like the best and easiest way to hack this into a power supply would be to trade for a power supply that doesn't have all the features you want, and then hack those features in.

author: ed_reardon
author_rank: Regular Contributor
timestamp: June 03, 2017, 07:36:50 pm
content: Hi again all,Sorry I confused the issue here, by turning it in to a supply I meant scrapping the amplifier entirely and just using the case, mains transformer and rectifier then building a PSU type board for where the amplifier board was: not modifying the amplifier circuit as such.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2017, 10:04:33 pm
content: In the other thread (for future reference please keep it to one per topic) you talked about it being early to mid 2000s, which means its quite likely you'll be able to modify the amplifier circuit itself into a DC power supply.