Yamaha A-1 amp conundrum ...

[reneborg]

We're talking about filtering the amplifier output to the L&R channels...like a crossover with no cross. A cap in series with the '+' terminal, and an inductor in parallel (for a 12db/octave rolloff), to mute some of the bass that the subwoofer is already handling.

No one suggested that this is the ideal solution for your speakers. Your choice of integrated amplifier has already rendered that option unavailable. But using high-quality film caps and inductors in the C-L filters for the L & R channels should give you results that are more than adequate. It certainly isn't the end of the world. All we're doing is rolling off the bass. No biggie.

If you want to use the speakers as they were apparently intended to be used (which seems to be an intentional PITA [sorry John Bau, but what the hell? :scratch2:]), then you need a separate preamp and amp, or a receiver or integrated with true pre-out/main-in jacks. If you want to use them with your current setup, see my previous post on the subject.

Edit: I took a look at the manual you linked to. It looks like the 'box' is a passive line-level filter to the main L&R channels. IMHO, this is a questionable approach, as amplifier input impedence specs are often not much more than educated guess (at least with BJT transistor inputs). Using this type of filter is not very accurate, to be kind. If you have a reasonable handle on the impedence of the L&R satellite speakers, you can probably do a better job (frequency-wise) with a high-level filter such as I previously suggested.

There are advantages to filtering at a line-level and feeding the filtered signal to the amp, but many of these advantages are lost by attempting to do it with passive devices at such low signal levels and varying amplifier parameters.

Thanks for more input on this! To help make sure I understand, could you please explain what a 'C-L filter' is? Also, can you please explain how this relates to your previous suggestion of "... rig up a 1st or 2nd order filter for these main L&R speakers to keep a bit of the bass out of them"?

It sounds like it's not such a big deal to filter the lower frequencies from the satellites. I'm sorry that I must seem dense, but the lingo is new to me!

The box, pictured above in this thread, has a selector to match the amp's impedence. I don't have any idea if this properly addresses your point, however :scratch2: If it doesn't, then what do I need to know to address this? The speaker's impedance is 4 ohms. I've pasted some specs below.

Oh, and I don't know if Bau wanted to intentionally make it a PITA, but the speakers really do sound very good, considering their cost and when they were produced. I guess I'd have to say their sound, to me, is worth their limitations. :tresbon:

Rene


Drive-units: 1" soft-dome tweeter, 6.5" pulp-cone woofer.
Low-frequency alignment: sealed-box Q=0.78.
Crossover frequency: 2kHz.
Crossover: high-pass slope, approximately first-order, 6dB/octave; low-pass slope, fourth-order, 24dB/octave, Bessel; both drivers connected with the same polarity.
Frequency response: 60Hz-17kHz (-3dB).
Sensitivity: 84dB/W/m.
Nominal impedance: 4 ohms (3.6 ohms min. at 4kHz).
Amplifier requirements: 25-100W.
Maximum power handling: 50W continuous, 100W peak.

btw,the crossover info here is all stuff I don't understand.
I do understand frquency response, sensitivity, and the rest. :yes: I know, that's not saying much.

 

[EchoWars]

A C-L filter is just as I described in my previous post...a capacitor in series with the leads going to the L&R channel, and an inductor in parallel. The capacitor has a falling impedance with rising frequency, and an inductor has rising impedance with rising frequency. One cap and one inductor with properly chosen values will give you a 12db/octave rolloff starting at your target frequency.

Let's say that the sub will handle everything from 120Hz on down (I do not know the specs on your sub). If you wanted your satellite speakers to do the work from 120Hz up, and roll off at 12db/octave from there on down, then you would add the C-L filter.

Here's an online calculator for a crossover. For a second order Linkwitz-Riley (I think the Linkwitz-Riley slopes are the way to go, but there are other opinions), you'd enter your L & R speaker impedence in the calculator where it says 'High-Pass' (4 ohms), and put whatever you want in the 'Low-Pass' section, since we're only making a high-pass filter and not a full-blown crossover. If you choose a Linkwitz-Riley filter slope, 120Hz, and 4 ohms for the high-pass impedance, then you end up with a capacitor size of about 166µf and an inductor of about 10.6mH.

The inductor is no problem, but that's a rather large cap if you want to stick with a film cap. But, still cheaper than buying another amp.

 

[msskks]

Someone please correct me if I am wrong (and I am a little late to the party), but I don't see an issue with creating a true pre-out/main-in connection in the int-amp from looking at the schematic. Don't use the 'pre-out' on the yammie that came from the factory- those are simply a speaker-level adapter that's mis-labeled. However, looking at the schem, I think it would be easy to tap in a couple pair of plugs to it like you wanted to in the original post. The 'Drive_chm' schematic- board NA07040- the connection on the left about 1/3 down on the board- is the preamp input to the amp section. This is where you need to tap. the center wire connected to the circle around the other 2 is the ground (probably a shielded wire internally) and the other wires are your left and right preamp level signal. It wouldn't surprise me if that connection is actually some type of detachable connector on the board, making testing quick and easy.

Mike

 

[reneborg]

Hmm, msskks. What you say is intriguing, but I certainly couldn't do the work myself and it seems folks with far, far more knowledge than mine suggest I don't do something like this.

To be honest, despite not doing the modification, I'd still like to see the area of the amp that you are describing for curiosity's sake. When I look at the schematic, even I can see the area you are pointing out. The trouble is, however, I don't know how to DO anything like this.

 

[dadster]

I agree

After reviewing A-1 schematic I agree. I haven't built a low-pass filter to directly drive speakers as is suggested, and that may be a workable solution, but if if you are seeking Spica nirvana it would seem to be sub-optimal (heh - sorry).

My advise would be to use an external amp and use your existing Spica-sub crossover as designed.

I got your PM reneborg, and I do not recommend doing anything to the A-1.

 

[Pete B]

I also am against modifying that unit.

Bau's crossover is first order, so if you're going to put one in series with the speaker it should also be first order. The manual says -3dB at 88 Hz.
The only issue with passive at speaker level is the rise at the fundamental resonance of the speaker - it is not purely resistive. What I suggest,
is to put a 25 ohm 25W resistor in parallel with each TC-50 system.

Then, we need a compromise resistance value, between the peak at resonance and the midband resistance of the system. I'd use 6 ohms.

Use Bau's formula: C uF = 1809/R or C uF = 1809/6 = 300 uF

The rise in impedance at resonance suggests that a smaller cap is required. If the peak is 10 ohms, then: 1809/10 = 180 uF

200 to 250 uF is probably about right.

You might purchase a 2 - 100 uF, and 2 - 50 uFs so that you can try 150, 200, 250, and 300 uF. Try it, see what sounds best.
Might start with electrolytics:
http://madisound.com/manufacturers/bennic/bennic-caps-electro.php

Perhaps you might want to use 50 uF of low cost poly caps, if you like the results:
http://www.parts-express.com/dayton-metallized-capacitors.cfm

 

[reneborg]

Wow, Thank you, Thank you, Thank you, Pete! :thmbsp:

Between your info and EchoWars have so kindly given me, it seems I need to try this out, though I admit it'll take me a bit more thinking through it to digest it all and make sure I understand it enough.

I'll work on this when I return from a little camping trip we're taking. I appreciate the help -- complete with sources even! :yes:

I might have another question or two later on, so I hope you don't mind if I bug you one more time if I need to.

All the best!

 

[cdfac]

i know this is an old thread, but i had a thought about why there weren't main-ins to go with the pre-outs. perhaps it has something to do with the fact that there doesn't appear to be any separate buffer or flat amp (assuming the tone controls are turned off)? according to the block diagram, the non-phono inputs run straight into the three-stage power amp section.

Stable High-Gain DC Power Amplifier with FET Input
You may be surprised to find that the power amplifier in the A-1 has about 20dB more gain than offered by conventional types. This is the reason, as stated before, for the simplified circuit configuration of the A-1.
It is extremely difficult in a high-gain DC amplifier to hold the DC drift down to negligible levels, but Yamaha has solved this by developing a special FET, matching a pair of it carefully for electrical and thermal performance, and using the resulting Dual Fet in a differential input amplifier for the DC power amplifier. This low-noise differential input is then followed by a current-mirror-loaded differential amplifier, an emitter-follower buffer amp and the final stage is a Darligton-connected SEPP-OCL DC amplifier.

maybe Yamaha didn't trust an external preamp to have low-enough DC?

 

[reneborg]

hmmm, interesting, especially when I don't understand >50% of the technical details here. But, what I ended up doing was using this A-1 as a pre amp. Took the signal coming out to the Spica crossover, then the pre signal went to an M-4 power.

Amazing phono stage on this is why I bothered using it this way. The quality of the phono section convinced me to find a Yammie preamp from that era, so I got a C-4.