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[Images Alert!!] Newbie here, a long intro, and a question about quad 306

T

tirthankar

Member
Hi All,

This is Tirthankar from India, and this is my first post over here.

A little background first.. sorry if you find it too lengthy.

I recently started on this audio journey by getting myself a very basic home theater system containing a denon receiver and a Polk sub-sat system.
Initially it was great, but when I listened to stereo songs on that system, I felt that there was more that can be had.

I decided to get myself a dedicated stereo, with a stereo amplifier and a pair of bookshelves. I listen primarily to classic rock(pink floyd), soft vocals and country (Diana Krall, John Denver, Neil Diamond, Don McLean, Don Williams and the likes), lots of instrumentals of various genres and western classical. Sometimes I also listen to modern pop and some metal (Metallica, Iron Maiden).
I started scouting around demoing systems available in my city in my budget. I listened to Mordaunt Short Aviano, Epos, Quad and Focal JMLabs speakers on CA, Creek and Denon amps. Though everywhere I was blown away by the level of detail and size of the music, I couldnt find any combination which could satisfy me. Except a Cayin Class A 40 W amp with the epos speaker. It was just right, smooth and detailed, velvety with no hard edges. But the amp was too expensive for me.

But I heard the best I had till then when a very experienced audiophile in my city kindly invited me to listen to his system, a heavily modded quad 606 coupled with a B&W 802 matrix. Those 3 hours I was in audio nirvana.:D
The gentleman advised me to get a quad 306 for an amp, and being a newbie that I am, after a long dilly dally of getting a used amp off ebay, I finally ordered one and it arrived today.

Now I plan to use this amp with a passive pre, with an audio pc with either essence stx or esi juli@ as the source. But before all that, I plan to carry this amp and preamp with me to all the stores I have visited and re listen to the speakers I have listened to.. and choose one.

Though the seller has assured that the amp works fine, may be I am too paranoid about carrying this to the shop and then blowing up a demo speaker :eek:

So as soon as the amp arrived, within minutes of unpacking it, I took some photos of its internals, and I would love you gentlemen to look into those photos and tell me what you think of the amp's current state.

The whole amp

29vjkhi.jpg



Close up on the PSU Caps

166jcd0.jpg



Speaker Outputs

2ll0y1d.jpg



Inputs

35itzx3.jpg



The rest

10cjitf.jpg



And the rest

axcy3a.jpg



Please let me know if you think there are some other close ups from different angles required for you to make a better decision, and I will be happy to upload.

Also, if needed, what do I need to test the amp and check that it works fine and wont embarass me during the demo or worse, make me a very sad and poor man :)

Thank you for going through my post. Wish you all a good day
 
It all looks very clean and tidy to me. The only real check you can do is to put a multimeter on the loudspeaker outputs and measure the DC voltage. It should ideally be zero, but up to 100mV or so is acceptable, although anything over 50mV is rare.

S.
 
Serge, do you not think those psu caps look suspect? There's a fair amount of rippling on the top there. Nice amp the 306, and a joy to work on.
 
No, I've seen new ones like that. I think that's just a plastic sheath, but can't be sure. Firstly, if you check the DC offset and it's OK, then the amp is safe to use on demo loudspeakers.

Then, if there's no or very little hum, and it's the same on both channels, I don't expect there to be much wrong with the reservoir capacitors. However, the only way to be sure is to measure the available power output as if the capacitors have gone low on capacitance, and/or high in ESR, the power supply volts will sag more, and the amplifier won't provide its rated output.

If you don't have the means to make the measurements, you could just change the capacitors, replacing like with like isn't going to be expensive, but I would avoid "audiophile" grade capacitors. Standard commercial grade will be fine. I can't see what the temperature rating is of the standard caps but as you're in India, it might be good idea to get some rated at 105 degrees rather than 85 degrees.

S.
 
I have a 306 and have serviced it along with four others in the past 12 months.

They are solid, reliable and transparent amplifiers that require very little by way of servicing to keep them tip-top.

The pics show an absolutely standard 306 in excellent condition, just as it will have left the factory. Much as Serge says above, just about the only thing that might require replacing are the electrolytic capacitors, especially as the unventilated 306 runs very warm at Idle, and can run hot when moderately stretched. No surprises there - it is a physically small amplifier.

The only items worth replacing are the four 47kuf PSU caps and the two small black electrolytics in the bottom pic. Change nothing else.
You can use 68kuf caps at 63v in place of those fitted which in combination with the choice of 105 deg rated caps will extend working life considerably.
I use good quality Panasonic low ESR caps which are reliable and reasonably priced.

I'm happy to provide part numbers if that helps.
 
Thanks a lot for your replies, Serge, James and Robert. :)

I had planned to change the PSU caps to 6800uf Panasonic, as recommended by Robert in some other thread, but that was not to come till I have spent 3-4 months with the system.
So as I understand, if the DC Offset turns out to be Ok, I will be safe without changing anything on the amp at the moment, right?

Let me start hunting around for a good multimeter. I will post back with the results.
 
Thanks to the Chinese, good DIGITAL multimeters are cheap these days. I would stay away from the cheap analogue, i.e. non-digital ones, since they have mechanical linkages that if cheap could be unreliable. Probably best to avoid the ultra-cheap DIGITAL models around 8-10 Euro and go for the 12-18 Euro ones. If you buy it in an electronics shop, rather than a general hardware shop, they should be able to tell you what model is good value for what you need.
 
tirthankar said:
Thanks a lot for your replies, Serge, James and Robert. :)

I had planned to change the PSU caps to 6800uf Panasonic, as recommended by Robert in some other thread, but that was not to come till I have spent 3-4 months with the system.
So as I understand, if the DC Offset turns out to be Ok, I will be safe without changing anything on the amp at the moment, right?Let me start hunting around for a good multimeter. I will post back with the results.
Click to expand...

Yes that's right. If the DC offset is OK, then you won;t do any damage to any loudspeakers you connect to.

S.
 
I have got two questions.

1. Can I measure the DC Offset of the amp without connecting a preamp? If yes, how to go about that? The reason is I dont have a preamp yet, I was planning on ordering it next month, an axiom passive pre.

2. I talked to an electrician guy I know to see if he can lend me a multimeter capable of measuring millivolts. He works on analog electronics and seems to know his stuff. When I told him I need it to measure the dc offset of a power amp, he went on to say a lot of things like there should not be any dc offset in a power amp because of feedback cucuits,m closed loops etc etc, most of which I could not understand as I have recently started on my reading :(
Anyway, he said that he doesnt have a millivoltmeter, but he can bring a milliammeter which can be connected across the terminals and the ground and if it registers a deflection, will show there to be a dc offset. (he was more or less confident there should not be any for a good amp).
Now he has got me confused and I have turned to you guys for your suggestions and whatever you can give me.:confused:
More specifically, I am pretty sure he is wrong about the absence of DC offset, the the way he said about measuring it, is that the right way?
 
Q1

Yes, there's no need to connect a pre-amp. All you want to know is whether the power amp itself has any offset.

Q2

Your friend is right that theoretically there should not be any offset, but in practice there will always be a small one, hence my comment previously that it should be below 100mV, and in practice is seldom above 50mV. If the offset is higher, it shows that something inside the amplifier isn't right, and should be looked at by someone who knows the circuit.

Using an ammeter to check for offset won't work, as it's too sensitive. As ALL amplifiers will have some small offset, so this will indicate on the ammeter, but unless you know a lot about the ammeter's resistance and therefore voltage sensitivity, you can't tell anything about the amount of offset. I really do suggest you buy an inexpensive digital multimeter, as it will be useful for many things, like checking cables, checking fuses, measuring the mains and so on. No home should be without one, and every HiFi enthusiast needs on at the very least.

S.
 
Serge is quite right with the above.
However to put your mind at rest, the 306 (and 606) does not have a centre tapped mains transformer from which the 0v point is taken. Instead, this point (referred to as a virtual or floating ground) is taken from a point between the two PSU caps per channel, with a small balancing network.
It is therefore very unusual to see dc on this range of amplifiers and it would indicated a capacitor short, which is extremely rare.
 
Thanks sarge, I did feel that the ammeter will not be worthwhile as it wont give me any quantitative idea about the dc offset, just an indication of its presence. Will continue my hunt for my own multimeter. Actually quite a few sudden liabilities has seriously depleted my funds and I am having to think about even spending 50-60 GBP.

Thanks for the post Robert. I followed the post and found another interesting read. Here's a quote
Note the TLC 271 op-amp in the 306 acts as a DC servo to remove offset from
the output and is not in the forward signal path though does come into play
as a sub sonic filter. The need for matched ( diff pair) input devices has
been thereby eliminated.

Another interesting feature is the lack of a PSU centre tap and deliberately
asymmetric DC supply rails ( see T11 & T12).
Click to expand...
The amp seems to be full of clever ideas to make it as bulletproof as it is now regarded to be.

337alant: the matched components are looking good mate. thats the 34 preamp and the fm4 tuner, right? what speakers are you using?

Thanks everyone.

Tirthankar
 
Sorry about lack of funds, but here in the UK one doesn't have to spend £50-£60 for a multimeter, they are available down to below £10, which will do the job, although spending perhaps £15 gives that bit extra ruggedness and adds additional facilities (like AC current, transistor testing, continuity buzzer)

S.
 
Okay, finally got myself a nice cheap DMM from a company called MASTECH, hope its good enough. Looks sturdy enough though.

Here's what I did.

  1. Connected the amp to the mains using a mains lead.
  2. Turned the amp on and let it run for around 5-6 minutes.
  3. Took my DMM, set the Voltage range to 200mv DC
  4. Touched the black lead to the black speaker terminal, the red lead to the red speaker terminal to get the reading. Did this for both the right and left terminals.
  5. Took another reading after 3-4 minutes.

Results: Reading of 00.3 - 00.4 for left channel, (-)00.1 - (-)00.3 for the right channel.

Too low meaning a faulty meter? Loose contact? Excellent offset performance? Or did I do something wrong?
What is your take?
 
Looks good to me. 0.3to 0.4mV left, -0.1 to -0.3mV right seems perfect. If you connect up a loudspeaker, I would expect it to work fine.

Good luck

S.
 
Long time since I have posted an update here. After auditioning speakers for two months I finally settled for a used pair of B&W CDM1-SE for 700 USD shipped. The pair is in great condition with no visible marks and they sound fabulous.

I auditioned Epos M12i, KEF q300, Quad 11l2/12l2, Focal 716V and B&W 602s2. The quad paired excellently with all the speakers, making me think that a 306 with a passive is a very neutral but musical amplification set. They easily beat any integrated I heard in the range I aid for them.

The B&W 602 s2 sounded best to me with the right combination of details and warmth, and when I saw a pair of cdm1-se in ebay I decided to take the plunge and get the pair situated above it in the range without auditioning.

I was not disappointed.

Thanks to everyone who has helped me in this thread and provided me the tips and encouragement to go ahead with this amp.
 
I too have always been confused by the decimal point that appears on DMMs. But I think your readings mean you have -1 millivolt on one channel and -3 on the other. Which is absolutely perfect, practically nothing.
Will someone more expert please correct me or confirm?
 
I'm no expert but this is my understanding. It looks in great condition, it's a nice amp, and I like the 306 almost as much as its predecessor the 405/405-2. I have a modified 405 in the spare room now, it's not going anywhere because if my valve amp goes sick I want something that I can just drop in.
 
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