Skip to main content

Altec 605B experiments

Hot on the trail from the 1" compression driver survey, I set forth to re-evaluate my open baffle mounted Altec 605B. This Duplex system is essentially an Altec 416A woofer + Altec 806A compression driver with an integral short horn. I wanted to address a mild honk and try to squeeze out more "air" in the top end from the horn/driver combo.

605B as a 416A woofer

Since my modus operandi has always been to listen carefully to each component as a separate entity, I disconnected the compression driver from the N1600C crossover and listened to the woofer rolling off at about 1100 hz as well as bypassing the crossover, in full range mode. 

After several days of listening with and without the crossover, I realized that the "honk" actually emanated from the woofer. Perhaps this was due to the typical midrange impedance rise in a woofer exacerbated in an open baffle by the lack of a true enclosure? So I winged and auditioned a few Zobel EQ values and ended up with 10 ohms + 11uf in parallel with the woofer terminals as the "best compromise." This circuit cost a couple of dB in efficiency but did not significantly affect dynamics while minimizing the honk. 

Towel damping

Not scientific, but hanging a towel behind the driver focused the sound a bit further ;)

+ Altec 32B horn

For kicks, I did another round of listening with the 605B acting as a woofer coupled to the Altec 32B horn using the same drivers (except the Beyma) and crossover options discussed in the previous survey. But unlike the 12" Altec 414A which has a smooth roll off characteristic, the 605B woofer had to be configured as outlined above. In this configuration, the woofer blended nicely with all four drivers. In fact the 605B woofer section + 32B/802D horn combo had a similar midrange tonal balance to the Altec 2-way. Could that be a 32B horn coloration? ;)

Altec 16 ohm diaphragms

The Altec 605B was factory fitted with the aluminum voice coil 20275 diaphragm. By the time I acquired it, the diaphragm has been replaced with the newer 34852 with a copper voice coil. I've swapped the 34852 and the 20275 (from my 802D) in and out of the 605B and always preferred the midrange oriented 20275 over the more extended high frequencies from the 34852. Thus, to my ears the 34852 and 20275 cannot be used as a stereo pair. Alas, NOS 20275s are very scarce but fortunately GPA is reproducing original 34852 diaphragms. Just to make sure my 34852 is an original Altec item I posted a query at the Altec Users' Board.

34852 vs. 20275

Having confirmed that my 34852 was genuine, I compared the JEL/N1600C XO to the stock N1600C in search of more sparkle and air from the 806A. The JEL/N1600C with EQ circuit did augment the top end a bit, but not to the same degree achieved by the other drivers with more powerful magnets used in the 32B horn experiment above. With or without the Zobel network, I found too much depression in the midrange of the 605B. This was most notable in the vocal range but not as audible with instrumental music. Switching to the 20275 diaphragm improved the midrange a smidgen but I was still not fully convinced that using the JEL/N1600C XO was the proper course.


Altec N1600C + Zobel EQ

For now I'll continue using the Altec 605B with the N1600C crossover with Zobel EQ + a sparkler...

Sparkler = Altec 3000H


  1. You might experiment with small bypass caps around the 5ohm resistor to add a little air. I'd probably start at 0.47uF and work up/down from there.


    1. Hi Roscoe,

      I think I've tried bypassing the 5 ohm with a small value cap but did not hear a boost in extreme top end performance. But thanks for reminding me, I will try it again.


  2. Hello

    Just wondering your Altec 32B horn plastic or aluminum?


    1. The one pictured above is plastic.


  3. Hello I'm trying to resurrect a pair of 605B's and am unable to find a circuit diagram of the N1600C crossover network. I have traced out the circuit , however I am unable to find any component values. I would appreciate if someone could post or email me, the CCT diagram or the component values as I would like to hear the originals before I start playing with other crossover networks
    Regards Gae

    1. The stock N1600C schematic with values is above. Just omit the Zobel network for the woofer (11uf + 10 ohms).


Post a Comment

Popular posts from this blog

Altec 2-way horn system redux

Truth be told, I was never fully satisfied with the performance of the Altec 2-way which is why I never uploaded the old article in the Arkiv.
Let's recap how the 2-way horn system evolved...

The project was conceived in mid 1998 because I needed a monitor system that was more efficient than the OB/755 so I could listen to SE amps with 2 watts output and below.
I commissioned my cabinet maker neighbor to build 614 bass reflex cabinets and loaded them with Altec 419A 12" drivers + 811 horns (early version/heavy sand cast) and 804A compression drivers gathered from hamfests and radio shows. Crossover was a textbook 1st order/6dB/octave hinged at 900 hz and the horn padded down to match the sensitivity of the low frequency driver.

Eventually I acquired proper 12" woofers - Altec 414As, which lack the upper midrange nasties of the 419As. The 414As have great midrange quality even if they lack the bass end of their larger 416/515 brethren.

802D compression drivers replaced t…

2017 Update: Hi-Fi Room

One year progress report

Garrard 301 + Ortofon RMG 309/SPU GME or SME 3012 + Denon DL103 Altec 4722 MC step-up
Digital line level sources processed  through UTC A20 transformers

JEL Stereo Preamp DX
JEL Stereo SE2A3 DX amp with Tango NY15S OPTs
Altec 2-way Altec 414As in 3.5 cf bass reflex enclosures + Altec 32A horns with 802D compression drivers
Mono Neat P58H 4 speed idler turntable + Velvet Touch viscous damped tonearm with Denon DL102 or GE RPX cartridges

je2a3 mono integrated amp

Stereo > Mono Line Level Mixer

Mixing line level stereophonic signals into full dimensional monophonic ;)
Here's a DIY project for mono aficionados that's so basic, I should have uploaded it many years ago. But other things got in the way.

Just like an MC step-up, these are entirely passive devices. 

The simplest way to convert stereo to mono is to connect left and right channels via a Y connector. But there's a more elegant way than just shorting two channels.

Resistors can blend two signals into one better by...

...using Allen-Bradley carbon composition for warmer sound ;)
We can end the mixing quest here but...

...the most elegant way of mixing two channels as was done in the studios by audio pros of yesteryears was through transformers.

This single UTC A-20 transformer was in a box lot of tubes and parts I picked up at a radio show. It's a high quality transformer designed for mic, mixer or line matching applications. Instead of trying to find a mate for MC step-up duty, I repurposed it for a g…