100v Line Audio Distortion - Page 1
Forum section: electronics / general

author: Arlen Moulton2
author_rank: Regular Contributor
timestamp: November 22, 2020, 05:23:07 pm
content: Hi All,I was recently gifted a TOA VM1120 100v and 70v line amp, it's one of their earlier models with a transformer output stage rather than direct coupling to the transistors. It's a 120w rated amp but I've been testing it with a 30w speaker, 16w speaker and 6w speaker (all set to 100v) but I've noticed what I'd class as an unreasonable amount of distortion to the sound, when bass notes hit they make a "pop" noise and the whole amp chassis judders, almost like an electromagnet is being energised. It seems to me like the output transformer core is being saturated, this happens above 25% volume (sound is crystal clear and rich below 25%) and is worse on the 100v tap than the 70v tap. I've noticed adding more speakers in parallel also reduces the problem but it's still there. Does anyone have any ideas what the cause might be, or whether this is what these amps are always like?Thank you in advance.

author: Benta
author_rank: Super Contributor
timestamp: November 22, 2020, 07:20:19 pm
content: Please clarify "all set to 100 V". Are your speakers real 100 V line speakers?

author: dmills
author_rank: Super Contributor
timestamp: November 22, 2020, 08:30:25 pm
content: That's probably core saturation right enough.100V amps in general are not normally designed to do bass, these are really paging system amplifiers, so think in terms of a couple of hundred Hz and up, the better ones have an HPF to enforce this given the limits of both the output iron and (sometimes more significant) the speaker transformer iron.

author: bob91343
author_rank: Super Contributor
timestamp: November 22, 2020, 09:44:28 pm
content: An oscilloscope would come in handy.

author: Arlen Moulton2
author_rank: Regular Contributor
timestamp: November 23, 2020, 07:13:44 am
content: Quote from: Benta on November 22, 2020, 07:20:19 pmYes, I have an Eagle PA speaker which can run either 8Ohm, 70v or 100v. I also have a pair of Bosch 100v ceiling speakers and another PA speaker with an RS Components matching transformer fitted.

author: Arlen Moulton2
author_rank: Regular Contributor
timestamp: November 23, 2020, 07:19:47 am
content: Quote from: dmills on November 22, 2020, 08:30:25 pmI've not had any issues with the speaker transformers (I've also tried running them directly from the output of a standard 8Ohm amp, they sound fine full-range) it's purely the one in the amp which is giving me grief. I've been using the Aux input which feeds directly into the pre-amp board, not any of the modules in the front bays, as 2 of these are for Mic only and the V-1054 is a bit characterful.Turning down the bass control on the pre-amp definitely helps, but there's still some distortion to voices and other high-frequency notes. Is this normal?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 23, 2020, 09:00:50 am
content: Depending on the circuit a defect of just poor adjustment of the amplifier could send some DC bias to the transformer and this way cause early saturation. The scope should be able to tel. I would also expect some adjustment / testpoints to make sure the bias currents are OK.

author: SpecialK
author_rank: Regular Contributor
timestamp: November 23, 2020, 11:38:05 pm
content: Well there's probably a reason the other guy gave it away....Have you had a peek inside? These things are often run 24/7 and this thing is quite old.

author: Arlen Moulton2
author_rank: Regular Contributor
timestamp: November 24, 2020, 08:01:59 pm
content: Quote from: SpecialK on November 23, 2020, 11:38:05 pmIt was given away because the suppression caps across the power switch had blown up, these not being necessary for operation obviously but I can't imagine your average person wanting to risk it after emitting smoke! I cut these out of circuit and checked the rest of the components, they all seem okay but I'll have another look around just in case. I ran the amp all day at full volume today with the 6w ceiling speaker and although it still had some audible distortion, it was very loud and the performance didn't decrease during the day.