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PMC TB2s - On The Way Out

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PMC TB2s - On The Way Out

Postby Bob Bickerton » Tue Aug 20, 2019 12:50 pm

I have, and very much enjoy, a pair of PMC TB2-A monitors, the active ones with the Flying Mole amplifiers, but unfortunately one of them is starting to play up. Crackling when it passes a signal on start up, but after a few minutes it settles down. Now I'm not that technically inclined, but I'm guessing there's a capacitor or other component on the blink, it does not appear to be a speaker cone.

Unfortunately PMC can no longer service this model as parts are unavailable, so I'm looking at options and PMC (helpful folk) have recommended an iNuke amp - which appears to be a Behringer model!

My two options are to use an external amp and keep the TB2s or look at alternative monitors.

My monitoring set-up is in a small room which has been professionally assessed and treated. I particularly like (and am well used to) the sound of the TB2s and I like that they maintain their quality of sound at lower levels. I work mainly with classical and acoustic music.

I happen to have a couple of (fan-less) Alesis amps hanging around, an RA100 and an RA500. I used the RA100 with my first set up which was a pair of (original) Alesis Monitor Ones but I'm not sure it, or the RA500, would be of a quality to match the PMCs.

So I'd be interested in hearing if anyone could suggest a suitable (fan-less) amplifier to run the TB2s.

As mentioned, the alternative would be to consider swapping out the PMCs and I've heard good things about the Neumann KH120s. Anyone care to give a direct comparison opinion?

As always - much appreciated.

Bob
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Re: PMC TB2s - On The Way Out

Postby Hugh Robjohns » Tue Aug 20, 2019 1:14 pm

Although the TB2s are getting a bit long in the tooth now, they are very good monitors indeed, and you have the advantage of knowing them and knowing how they work in your room. Switching now would involve a new learning curve as well as the expense, and while the KH120s are extremely good they are obviously smaller than the TBs and don't have the same effortless headroom and low-end power. While I doubt these are important requirements to you, personally, it may be a consideration for your clients.

Sadly, the Flying Mole amps are discontinued and obsolete, and almost impossible to mend -- although I have seen some people advertising repair services specifically for them on the web. And while PMC made their own Hypex-based mono amps for the TB2SAiii.... they don't any more... and even if they did, they wouldn't fit the slots for the Moles anyway!

However, the good news is that the TB2A is a powered speaker, rather than an genuinely active one, and so it's a simple job to wire in an external power amp in place of the Moles -- as you've obviously realised.

The TB2 doesn't present a difficult load, and its nominal impedance is actually around 6 Ohms so you'll get a little more out of most amps than their 8 Ohm rating would usually suggest. In my experience, I'd say pretty much any decent amp will do the job, depending on how much headroom you need.

I've run my previous early-model TB2s on everything from a 50W Rotel hi-fi amp to a Bryston 4B, via Quad 405 and 520s, and Bryston PP120s at different times...

I was bowled over by how good then sounded on the Bryston 4B -- they really do benefit from a lot of power and control behind them -- but they also delivered perfect usable mix-room performance on all the above, including the Rotel provided the monitoring level was sensible...

So I'd have thought it well worth trying the RA100 and seeing how you get on with it before looking around for other higher-quality options. I don't know what the second-hand market is like in NZ, but any of the decent pro studio amps or sensible hi-fi amps would do nicely.

Here's another off-the-wall thought... the later TB2As used a Speakon connector, but the early ones used the bi-amping quad 4mm binding posts. I'm not sure if the Speakon wiring maintains the separate woofer/tweeter access... but if it does you could bi-amp the speakers using both your Alesis amps, with the RA500 powering the woofers, and the RA100 on the tweeters! :-) (Edited: I got my power amps the wrong way around -- beefier one on the bass!)

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Re: PMC TB2s - On The Way Out

Postby Bob Bickerton » Tue Aug 20, 2019 1:22 pm

Thanks Hugh.

Yes, I’m definitely thinking of holding onto the TB2s, if I can, so I guess it’s a question of trying out the amps. They appear to have the speakon connection between the amp and the speaker - time to take them apart!

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Re: PMC TB2s - On The Way Out

Postby Hugh Robjohns » Tue Aug 20, 2019 1:28 pm

It's been a while since I've seen TB2s with the Mole amps, but I have a feeling they had a flying cable from the internal crossover board brought out through the rear port foam, terminating in a right-angled Speakon for connection to the amp...

If so, you should be able to connect another speakon to that from your selected amp...

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Re: PMC TB2s - On The Way Out

Postby Bob Bickerton » Tue Aug 20, 2019 1:45 pm

Yes, that appears to be the case.

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Re: PMC TB2s - On The Way Out

Postby James Perrett » Tue Aug 20, 2019 2:02 pm

Those symptoms sound very much like a dry joint to me. I'd take a good look at the circuit boards and maybe get the soldering iron out and try re-flowing the joints to the audio connectors and anything else that might become warm in use.

If this doesn't work and you don't mind a little DIY then I'd think about removing the innards from the Flying Moles and replacing them with a Hypex PSU and amplifier module. This might require a few extra small holes in the back panel but Hypex can also supply ready made wiring looms so there's very little work involved if you re-use the existing connectors.
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Re: PMC TB2s - On The Way Out

Postby Zukan » Tue Aug 20, 2019 2:17 pm

Would an ageing Bryston 4B help?
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Re: PMC TB2s - On The Way Out

Postby Hugh Robjohns » Tue Aug 20, 2019 2:22 pm

I'm sure it would... but the cost of shipping such a heavy beats to NZ would be disturbing! :-D
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Re: PMC TB2s - On The Way Out

Postby Martin Walker » Tue Aug 20, 2019 5:10 pm

James Perrett wrote:Those symptoms sound very much like a dry joint to me. I'd take a good look at the circuit boards and maybe get the soldering iron out and try re-flowing the joints to the audio connectors and anything else that might become warm in use.

Yes, that was my fist thought too James. I'd second having a close look at the offending circuit board with a magnifying glass and re-flowing any dodgy looking solder joints.


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Re: PMC TB2s - On The Way Out

Postby Hugh Robjohns » Tue Aug 20, 2019 5:51 pm

It is a very good point...

Although thinking about it, most Speakon connectors have screw terminals, so it be even worth checking whether there's a loose screw terminal in the connector?

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Re: PMC TB2s - On The Way Out

Postby Wonks » Tue Aug 20, 2019 6:41 pm

Can we just double check that you've swapped the input leads round, so that you know it is that speaker that's to blame, and not a problem with the source? I'm sure you have, but I thought I'd ask as it hasn't been mentioned.
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Re: PMC TB2s - On The Way Out

Postby Bob Bickerton » Tue Aug 20, 2019 11:27 pm

Thanks everyone - Yes I've isolated it down to the speaker.

Definitely sounds like an electrical component issue rather than just a a loose something or other.

Time to roll up my sleeves and put it on the bench!

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Re: PMC TB2s - On The Way Out

Postby ef37a » Wed Aug 21, 2019 6:42 am

Hi Bob,
I was going to suggest one of the D.Self designs until Hugh mentioned shipping to NZ!

I have been reading Self's book on power amps and the designs seem to me to be about as close to perfection as modern technology can get! Distortion up to around 3kHz is at the noise floor of the superb AP test gear and still better than 0.01% at 10kHz and 50W 8R. (6R would give around 60W)

The designs are quite complex once the various protections are added so I don't suppose they are cheap!

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Re: PMC TB2s - On The Way Out

Postby Bob Bickerton » Fri Aug 23, 2019 5:08 am

Thanks again for everyone's help.

Just a quick update...... I can't see anything obvious in the Flying Mole amp but I'll get my high-tech friend to look at it when he returns from extended leave.

In the meantime I've hooked up one of the Alesis amps. The RA500 (150w into 8ohms) seems OK but suffers a little self-noise. Unfortunately the RA100 suffers audible transformer hum. Either way I have a solution (sort of) until I can review the Flying Mole amp.

......and there's always headphones :lol:

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Re: PMC TB2s - On The Way Out

Postby Hugh Robjohns » Fri Aug 23, 2019 9:58 am

Well at least that proves the speakers themselves are okay, which is good news. Time to keep an eye out in the second-hand ads for a decent (quiet) power amp then...

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