DC on speakers - Page 1
Forum section: electronics / repair

author: MarkoGVA
author_rank: Contributor
timestamp: March 10, 2024, 08:41:06 pm
content: Hi all,I have a vintage amplifier (SAE Mark IVCM) from the 1970s, on which I have a transient voltage of 2.2 VDC on my speakers at turn-on. Then it immediately drops to 200mV on one channel and 60mV on the other.Is this likely to damage my speakers?Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2024, 08:45:55 pm
content: The transient is most likely due to the AC coupling capacitors charging up.200mV is only 5mW into an 8 Ohm load.

author: MarkoGVA
author_rank: Contributor
timestamp: March 10, 2024, 11:49:51 pm
content: So what would be the maximum speakers can take without damage?

author: Smokey
author_rank: Super Contributor
timestamp: March 11, 2024, 03:25:37 am
content: Quote from: MarkoGVA on March 10, 2024, 11:49:51 pmIts just a power dissipation issue right? A speaker is just a voice coil, so figure out what wire gauge is in the coil and then figure out how much power it can dissipate considering the winding thickness and insulation properties.Pretty much don't melt the wires and you are fine.

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2024, 01:27:20 pm
content: Quote from: Smokey on March 11, 2024, 03:25:37 amDamage is only one issue. DC will bias the voice coil away from it's normal quiescent position, which will cause it to clip one side of the waveform at a lower power level, that it otherwise would, gvein no DC. Whether it's an issue or not, depends on the speaker and how it's used.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 11, 2024, 03:00:48 pm
content: add a timed / delay relay for the speakers ?or you have some kits who offer dc protection tooeven 200 mv could trig the protection .... muste some problem(s) in the biais section ?? or recalibrate if its possible

author: linux-works
author_rank: Super Contributor
timestamp: March 11, 2024, 03:23:16 pm
content: I designed a relay timed switch for my amps. its a reasonable approach. mine is arduino and wifi based so you can remote control things easily. (if you want the source, pm me)

author: themadhippy
author_rank: Super Contributor
timestamp: March 11, 2024, 03:34:45 pm
content: Quotegreat for avoiding farts and squealing on power up/down but not so good when the output stage has had enough and chucks one of the supply rails up yer driver.

author: bdunham7
author_rank: Super Contributor
timestamp: March 11, 2024, 04:47:51 pm
content: It sounds pretty 'normal' for this amplifier and is not likely to damage normal speakers. These amps have a direct (not capacitor coupled) quasi-complementary output and do not have a protection relay nor any DC offset/bias adjustment IIRC. I think you can modify them to have a trimpot bias/offset adjustment instead of relying on select parts but I don't know the particulars. 1972 was still early days for solid-state audio power amplifiers, so they're relatively crude compared to models that came 5-10 years later.

author: donlisms
author_rank: Frequent Contributor
timestamp: March 11, 2024, 10:37:57 pm
content: 2.2v^2 / 8 ohms = 0.6 watts. Meh.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: March 11, 2024, 10:56:57 pm
content: DC offset of 60mV seems high to me but with no adjustment it may be passable.200mV is not, their is a problem, slight mismatch of parts, changed value part, high leakage - something is wrong.Jeff

author: floobydust
author_rank: Super Contributor
timestamp: March 11, 2024, 11:47:51 pm
content: Do we have a schematic for this SAE Mark IVCM?That much DC offset is usually problem for cross-over distortion because the idle/bias current ends up going to the loudspeaker instead of the output transistor pair. 600mV is 25mA at 8 ohms that the complement is not getting. It depends on how bias current has been set up, always done with no load present.That much DC offset and it moving around, I would check the input and feedback electrolytic capacitors, they may be old and dead. It would also affect the amplifier's bass response if the caps go low value, but not offset.I had one power amp the (NFB to GND) cap shorted and there was a big thump on power up and excess DC at idle.

author: p.larner
author_rank: Frequent Contributor
timestamp: March 11, 2024, 11:50:52 pm
content: if it was mine i would check the pa bias+possibly replace any coupling caps.

author: Mwapy
author_rank: Newbie
timestamp: March 11, 2024, 11:56:55 pm
content: If the speakers are connected (4 or 8 ohm, not sure about others) there should be no more than a few mVDC, 10 tops. Possibly a leaky cap, or bad tube if it is that old. When speakers are not connected, it's not a valid measurement.

author: langwadt
author_rank: Super Contributor
timestamp: March 12, 2024, 12:06:06 am
content: Quote from: themadhippy on March 11, 2024, 03:34:45 pmsame relay can and often is used to disconnect the speaker if that happens

author: bdunham7
author_rank: Super Contributor
timestamp: March 12, 2024, 12:58:12 am
content: Quote from: floobydust on March 11, 2024, 11:47:51 pm

author: floobydust
author_rank: Super Contributor
timestamp: March 12, 2024, 01:37:54 am
content: Thanks for digging that upIt has no trimpots for adjusting bias or offset. I wonder if they used hand-selected transistors/diodes.For the age/era of this and only 5 small electrolytic capacitors per channel, I'd outright replace them all.The NFB capacitor that shorted (in a different amp I was fixing) was in the same position as the 250uF 16V, it was those crappy Philips blue axials. Any leakage on the 15uF 35V part would also cause DC offset.Other than this, then a mismatch on the 2N4036's could be to blame, especially if another tech was in here.You'd have to make some voltage measurements to see where in the amplifier things are not right. It can also mean a transistor is damaged or half dead, which I have seen in another SAE amplifier, it showed up as grit during certain passages. Best to replace them before they outright fail IMHO.Adjusting bias, the VAS runs pretty high current here and I guess the 10R resistor or power resistors could be trimmed.

author: CaptDon
author_rank: Super Contributor
timestamp: March 12, 2024, 02:07:25 pm
content: The 2N4036 pair should clean up the D.C. offset. With a + / - 53 volt supply seeing up to 100mv across 8 ohms is kind of typical (usually less however) for an amplifier that is direct coupled and has a lot of age. If that amplifier can sustain + / - 50 volts at full output into 8 ohms you could have 312.5 peak watts at clipping or around 156 RMS watts at clipping. Desiring 10dB of headroom for musical transients that would equate to about 15 watts of average listening level with ample headroom. Still bloody loud even though the number looks small. Most of those old amps only contained a bias adjustment. Offset adjustments were very seldom seen!! The current mirror is supposed to compensate any offset but components drift with age. Although SAE were expensive they were still only middle of the road quality. I have seen plenty of them on the repair bench.