Turn off the preamp but leave the power amp on. Does the speaker pump now? If not, I suspect you have an unstable preamp that is defective.
I do not know if your speakers were damaged by the initiation of the preamp but the cone movement of the speakers does not suggest it since they cannot move on their own.
Kr4 - Thanks for the quick response!
I tried your suggestion - when I turn the preamp off but leave on the power amp the speakers don't pump!
Could this be a faulty tube issue? The minimax uses a 6x4 rectifer and two 12au7.
Could be the tubes but, if it happens on both channels, it must be something common to both channels. Since the pre is new, why not contact the company about this?
Kr4 - Thanks. I'm in the process of getting back in touch with the seller and hopefully will have this resolved soon and haven't suffered any damage.
It has been suggested to me that maybe the preamp is leaking DC to the amp which supposedly happens at low frequencies which would explain the slow cycle of the pumping. How does this sound to you?
I had a similair problem after picking up a defective BAT P5. I believe the problem turned out that it was generating a high frequency DC signal. When it was connected to my tube amp, it started over loading my power tubes. I then switched to a solid state amp and I noticed that my midrange drivers started pushing way in and out. The bass drivers were stable.
BAT put a filter in the circuit and that problem was solved.
I was told that it could be the tubes but in my case it wasn't. Hope you can either get some other tubes or find someone that can repair the preamp. I'm assuming that you tried disconnecting any inputs to the preamp. You made need to get the schematic.
Also, it didn't damage my speakers although I was concerned too.
Hi Steve. Thanks for the reply. I haven't tried disconnecting inputs to the pre. What would this accomplish? Is this to eliminate my CD player as the possible source of the DC?
Just to make sure & prove that the pre amp is the culprit. You never know how a combination of equipment can cause a problem.
Kr4 made an excellent point about both channels and that the problem could very well be DC leakage. That was the problem I had.
What is high frequency DC, anyway?
I think the problem is with the tubes, but not because the tube is going bad, but because it's a low quality brand.
When I got my tube amp, I've experienced similar problem, but I could also hear the modulation that appeared at approx 1 cicle per second. I replaced a pair of GE 6k7 tubes with a pair of RCA 6k7g and modulation stoped.
Try a different brand for a pair of 12au7.
I was told that the high frequency, that was out of audible range and that is why my midrange drivers were modulating as opposed to the woofers. The DC I believe is direct current as oppossed to AC. My problem was related to electrical components not tubes. I did swap to the tubes first to prove that they were OK and hopefully you have a source to try that.
Thanks for the suggestion. I just tried replacing all tubes with better NOS and no change in the situation.
Check the line voltage because if it is too low, the power supply may be in the pre may be oscillating. It could also be DC leakage.
DC would push the cones one way or the other. The only way DC could cause a cone to move back and forth would be the DC alternating between positive and negative like switching the leads on a battery.
For example, if you take a "D" cell and connect the positive to the positive on the speaker and the negative to the negative, the cone will jump out and stay. If you reverse the negative and positive, the cone would go the other way and stay.
As a matter of fact, DC does not have frequency. It doesn't alternate like a sign wave(AC.) It travels in one direction. An amplifier produces an AC signal to the speakers. For DC to cause your problem, how did it get through the amp. I would also think that you would be hard pressed to get enough DC through your amp for it to do this. If the amp has a DC servo, it would block the DC to the amplifier input stages. If it uses an input cap, it would block DC.
I think you have a very, very low frequency modulation problem(modulating AC)coming from one of the preamp amplification stages. I would contact the manufacturer.
Just to update . . . I removed the CD player from the loop and this had no effect. It's looking like it's the preamp.
Bigtee - thanks for the post. I don't know enough about electronics to have an idea about DC vs. AC but I will definitely bring it up when I speak with the seller.
Thanks to all who posted.
I'm pretty sure it's the preamp and the person who sold it to me seems to agree. This one's going back and a replacement is on the way. Thanks for all the suggestions and insight.
I've no idea what's high-freequency DC(probably something brand-new in EE) but BAT has tendency to drain some DC at the output.
Probably the same story with your pre which isn't neccessary because of bad tube...
DC ??? absolutely not. High frequency DC...what is that ??? Please tell us how high frequency it is, DC with frequency and still called DC ???
What you are experiencing is subfrequency motorboating, or, in other words, an unstable power amp that is probably only marginally stable with the speakers you are using. You might have been using a pre with a higher output impedance and just got away with it. However, with the low
Z of the EE MM, that puppy becomes likely unstable.
Anyway, w/o knowing what you are using, it's just my logical conclusion, not gospel...
The output impedance of the EE MM is 1.5Kohm which is high for a tube pre if I'm understanding this right. My previous pre had a LOWER Z and I had no problems matching with my Belles SS amp (100 Kohms). There is definitely something wrong with the pre. (Speak to Bill about this. The Belles is solid state.)
Sorry if I used the term High frequency DC out of context. I don't consider myself an electronics authority and I was attempting to interpet the explanation of a similair problem that I had.
Just to clarify, I had a problem with a phono stage that caused very unusual problems with my system that had baffled the manufacturer and was the most frustrating audio situation that I ever encountered.
The phono stage, a BAT P5, connected to a BAY VK 30se, caused the power tubes (KT-88) in my amp to light up. I didn't even play a recording and this would occur. I then used a solid state amp and instead of the amp reacting, the problem manifested itself in my speakers where the mid range drivers extended way out then retracted as Tooter has experienced. Note that the woofers didn't move.
I was told that it must be the amp, the tubes, the interconnects, the power conditioner etc.. I checked everything but the problem kepted happenning. At the time, BAT didn't believe that the problem was coming from the P5, so I sent the P5 to a 3rd party & they said the there was an excessive DC leak out of the P5 and they called BAT to convince them. Additionally, I was told that since only the mid range drivers reacted and that no sound was heard lent itself to a frequency that was inaudible.
Anyhow, a DC filter was put on the P5 and the problem was solved. BAT had never seen anything like this at the time, which was 2 years ago.
Fortunatly, Tooter is getting a replacement and hopefully this will be a happy ending.
I still cannot think of a clue there is anything wrong the EE MM but the power amp. It is because any amp that would have a problem being driven by any source having a Z=1.5k isn't worth its design...
Ykm - I don't think it's the amp. The person who sold me the EE MM has a dealer who pairs this preamp with the SAME power amp, Belles, that I am using.
Just to update the situation - the pre was sent back and tested. According to the seller/distributor there is nothing wrong with the unit so I am back at square one. I've decided not to go with a minimax and got a refund and will start looking at other options. Just to be extra safe I am sending my speakers back to Tyler Acoustic to have them checked out and will probably do the same with the amp.
I checked the line voltage and it seems okay. I should qualify this statement with the fact that I bought a line voltage meter at Radio Shack. I don't know how accurate these things are but after testing my outlet at different times of the day it is within acceptable range. 120-126V.
Same power amp, but was it the same speakers too ?
When I first put the Minimax preamp into my system there was a very loud hum and some static pops coming through the speakers
Amazing that the pre tested OK.
Re, testing: Rather than the speakers, shouldn't you be sending the amps for a check-up??
My speakers are middle of the road in terms of driving them (88db). They are basic 2 way monitors, nothing exotic. I've used different pres in my system and never had problems. I am convinced that the issue is with the pre, but I will be sending my amp for a check up just in case. What kind of speaker conditions do you think would cause the problems I'm experiencing?
It IS amazing that the pre tested ok. I have my doubts that this is really the case. My Belles amp will go back for a check-up soon. David Belles is out of town for a few weeks so until then ...
For those of you still following this thread my speakers checked out okay. No damage or problems.
Thanks to all for the suggestions.
Tooter, I was trying to find out any possible clue so to ask the speakers too. If the pre was "causing" your power amp motor boating, the amp is marginally stable. There are many thousand pres out there, the pres you used did not cause any problem with your power amp does not mean it was the EE MM if the power amp was marginally stable. This is not wrong because a hifi equipment designer would sometimes want his gear to deliver better sound with some trade off of slight stability or output power or heat generation or.... An engineer finds a power amp motor boating will first of all fix the power amp. Apologise if I have made some agressive assumption before knowing all the designs of your gears.
No offense taken! No need to apologize. I am just not clear on what you mean by power amp "motor boating" and "marginally stable". Can you elaborate? What part of the amp's design is deficient which would lead to it being "unstable." I'm not an engineer but Belles amps have gotten great reviews and I've used them with many speakers and pres without incident.
Tooter, first of all gears got great reviews do not necessary mean their spec. are perfect. I understand EE MM also got many great reviews too. Motor boating is some low frequency oscillation and that you saw the woofer was pushed in and out. If the oscillation is "faster" and "louder" in your case, you would hear it like the sound of an old motor boat and that's how it came the term. A few things can cause such oscillation, incorrect amount of nfb, decoupling caps do not have enough value, bad design of the PS...I am not a real expert, but have diyed quite many amps. Audio gears are very different from other electric appliances, spec. is one thing, but sound performance is the most concerned. There many many many audio gears in the market. Every engineer does them in a different way. You cannot expect when EE made his gears and when Belle made his gears to be able to match the rest of the world. If you search around, there are many many many sayings such and such pres don't work with such and such amps etc. But when you check them, they are all fine. As soon as you connect them together, they are "loaded" and affected by each other, say DC leakage, impedance...etc. So at the end of the day, your sepckers, Belles and EE MM are all well checked okay. That's all the fun of playing with audio I look at it.