Opinions on Double Advent Rebuild Please

[Pete B]

On 2/6/2019 at 11:53 AM, AustinJoeC said:

Pete, if one were to simply add some series resistance to keep the impedance above, say, 3 or even 4 ohms, does that open up the universe of acceptable power amps that could drive the Double OLAs?  (Recognizing that you’d need a more powerful (?) amp.). If this makes sense, any suggestions of a strategy to implement?

Joe

I take it you are not talking about a cap just in series with the tweeter cap but, 

rather in series with the entire system or system pair?  I really can't recommend 

this.  I have to say either don't run a parallel pair or get an amp/amps that can

handle the load.

If you change to film caps then yes use the ESR cap to keep the impedance dips

the same as original and not worse.

[Guido57]

I believe there is a technical/theoretical reason behind Pete's recommendation to not just add series resistors.  Pete knows a lot more about this stuff than me.  My point being there are some speaker 'selector' boxes out there that claim you can run multiple pairs of speakers with no impedance problem but for the most part they just add series resistors inside a black box.  Just beware of these items.  (Anything more sophisticated than that quickly gets expensive.)  My (limited) understanding is while this may protect the amp it is not good for sound. 

I've run stacked Advents with an Adcom 535 which I believe is 60 w/c without distortion, but it was just too much speaker for the medium sized room they were in so I never really pushed the system.  535's can be found for under $150 but newer models, anything with a suffix after the 535, go for more.

[AustinJoeC]

Thanks All, yes this was exactly my thought: run some additional resistance in series with the parallel pairs to keep the total resistance above a certain minimum level, thereby opening the options to use amps that are don’t have the power supply to run at low impedance.  (Correctly stated?)

I totally understand why you would add resistance when changing out the crossover caps so as to maintain the design ESR.

But why would adding a series resistance to the entire system be a bad thing (other than throwing away some potential amplier gain?)  Is it because, since the impedance varies with frequency, the addition of a fixed resistance will affect the different frequencies to a varying extent and unbalance the sound reproduction?  Or something else?  It may be a little academic, but it helps me think about how these systems actually work.  

[Pete B]

The question is how large of a resistor do you plan to use?

Yes, it forms a voltage divider with the non-uniform impedance of the speaker and 

therefore alters the frequency response depending on how large of a value you use.

It is as if you had a power amp with a lower damping factor value.

[AustinJoeC]

So I guess I had in mind a 2 ohm resistor, something that would keep total resistance above about 4 ohms.  Sounds like the OLA tweeters in parallel are less of an issue because of their higher DCR, so this 2 ohm value comes from the minimum in impedance of the woofers, 5 ohms @ about 100 Hz,  where with two woofers in parallel, 2.5 ohms.  So with the additional 2 ohm resistor, the minimum would then be 4.5 ohms at the ~100 Hz minimum.

So does this mean that the output at ~100 Hz is roughly halved because of the almost doubled resistance?  Whereas at frequencies with higher impedance, the output reducing effect of the additional 2 ohms is reduced because it’s a smaller relative jump in impedance?  If yes, then I can see where arbitrarily adding resistance might not be a very good idea...

Guess I’ll be putting a high current amp on the birthday list!

[ajfink]

2 hours ago, AustinJoeC said:

So I guess I had in mind a 2 ohm resistor, something that would keep total resistance above about 4 ohms.  Sounds like the OLA tweeters in parallel are less of an issue because of their higher DCR, so this 2 ohm value comes from the minimum in impedance of the woofers, 5 ohms @ about 100 Hz,  where with two woofers in parallel, 2.5 ohms.  So with the additional 2 ohm resistor, the minimum would then be 4.5 ohms at the ~100 Hz minimum.

So does this mean that the output at ~100 Hz is roughly halved because of the almost doubled resistance?  Whereas at frequencies with higher impedance, the output reducing effect of the additional 2 ohms is reduced because it’s a smaller relative jump in impedance?  If yes, then I can see where arbitrarily adding resistance might not be a very good idea...

Guess I’ll be putting a high current amp on the birthday list!

The short of it (no electrical pun intended):  if you're willing to get a powerful amp, just make sure you get one well-built enough to handle it and don't fret over the resistors.

One of the reasons Pete has mentioned adding resistors to these this in the past was to make up for the lower DCR of newer poly caps often used to replace the old electrolytic ones.  In those cases, they would be relatively low-value resistors, and they would only be in line with the tweeter.  To some people/in some situations, dropping the mids and highs down a notch (again, no pun intended) is a good thing anyway (see:  BSC).  Adding a resistor between the amp and the whole speaker also affects the woofer, too.  The woofer consumes far more of the energy than the tweeter, though....

 

[rdenney]

A very late hit to an old thread, but for posterity (and future searchers just like me):

I'm running stacked Advent NLA's using two identical amplifiers, B&K Reference 125.2. These are conservatively rated at 125 wpc. While one would probably handle the Advents in parallel if not driven too hard, and despite that these amps are high-current designs, the MOSFET finals are not really happy with speaker load impedance dipping down into  the 2's, so I decided to use the additional amp.

But when feeding two amps, phasing is important. If you use Y connectors from a preamp--no worries. But if you use two outputs from preamps that have multiple outputs, be aware that some are phase-delayed, especially if they are routed differently through things like tone controls. Also, it's best to run identical amps so that there are no subtle phase cancellations.

I'm running one amp out of the "bypass" outputs on my Adcom GFP565 preamp, and the other out of the "lab" outputs. One of them runs through the tone controls and the other does not. The preamp has a tone defeat feature, and when the tone controls are defeated, everything stays in phase and the system sounds extremely good even with my suboptimal speaker placement. When the tone controls are not defeated, phase cancellation gives the same effect as one channel being wired backwards.

When working on Advents, make SURE the drivers are wired in the same phase as they were originally. It's not always obvious.

I used foam rolls from Midwest Speaker repair for all four woofers, and I did have to source three new NLA tweeters, including one for my original pair of utility cabinets which I bought in 1977. One woofer in the deluxe cabinet pair I bought more recently had been "refoamed" prior to purchase, but the voice coil was dragging at some frequencies, so be aware that the effects of home improvement are not uncommon with used Advents. 

I replaced all my crossover caps with Dayton Audio precision caps bought from Parts Express. I did not change values, and the BSC did not sound better to me.

Rick "for posterity" Denney